ENH: DataFrame isin
WIP for now.

See #4211 for more info.

A few things to check before this is ready to merge:

This is basically a convenience method.  I use the `Series` method for each column passed, and aggregate the methods sensibly.

A few questions before merge though:
- Do we have a preference for `any`/`all` vs. `and`/`or`?  Seems like `any/all` would be more consistent with other methods like `df.dropna(how='all')`
- I'm not thrilled about all the nested if statements.  The problem is accepting both dicts and just flat arrays as values.  The idea is to use dicts like `{colname: array_of_possible_values}` if the set of possible values to match against differs by columns (most likely case I think).  I need the extra nested if statement here:

``` python
        if how == 'and':
            if isinstance(values, dict):
                cond_n = len(values)
            else:
                cond_n = len(self.columns)  # Flat matching.
        elif how == 'or':
            cond_n = 1
```

since the two ways of calling will be a bit different.  If you're passing a dict it will probably just be df.isin(values=dict).  If you don't care which column matches what, the call will be df[subset_of_columns].isin(values).  It's messy, but it works.
- I check for empty values, and raise an error instead of just returning all `False`s.  Passing an empty list as values, it would actually would work.  `Series.isin([])` will return all Falses.  But it would fail on an empty dict, so I just check for it ahead of time and raise an error.
- Also, I may have messed up the history.  I thought I rebased and squashed everything into one commit, but apparently not.  I had some merge conflicts that I had to fix (just picking HEAD over the old stuff everywhere). I'll take a closer look at this.  Also I may have done `git pull` instead of `git pull --rebase` at some point.  Will that mess up the history?

Happy to move this to .13 also.

would an `axis` parameter be useful? i think it would be more consistent with the rest of pandas
 you should remove that merge commit

``` sh
# if you haven't added a pandas master remote
git remote add upstream git://github.com/pydata/pandas.git

# fetch latest changes
git fetch upstream
export EDITOR='gvim -f' # or emacs or whatever you want
git rebase -i upstream/master
```

after that you can hit the `f` key (`fixup`) or `s` key (`squash`) when the cursor is over the commit you'd like to fixup or squash
 what i like to do when i'm close to finishing a PR and before I add release notes is 

``` sh
git fetch upstream
git rebase upstream/master
```

in fact i have `alias gfrb='git fetch upstream && git rebase upstream/master'` in my `.zshrc` the mnemonic is "git fetch rebase", it's not super general, but i pretty much name all of my upstreams `upstream` so it works in most cases and if it doesn't then there's always `git reflog`

this puts your changes right on top of the latest in `master` and generally makes things easier when you want to merge. you can add release notes before doing this but it most likely will cause a merge conflict (not a big deal, but still annoying)

check out the [pandas wiki](http://www.github.com/pydata/pandas/wiki) for some more git and dev tips.
 I really think that the default behaviour should apply across every element (i.e. not be row-wise), like naive ~~(incorrect)~~ implementation:

```
def isin(self, items):
    return self.applymap(items.__contains__)
```

Can always then apply all/any and think it's probably easier to read:

```
df.isin(blah).all() # possibly setting axis
```

Saying that, if it can be more efficient to do the row-wise behaviour, having that as an option is a good thing. Though not entirely sold on 'and' and 'any' as argument values (but perhaps these are used elsewhere in codebase)...
 but +1 for isin as method, good addition
 i could also see this taking a dict of columns => sequences
 nice `dict` is already there
 Also, does this work for DataFrames with repeated cols/index (think you may get bit here)?

I'm afraid the naive implementation may be the easiest (and make most sense here)...
 @TomAugspurger lib has an ismember cython function (which is what Series.isin uses), so this way is pretty fast):

```
def isin(self, items):
    return pd.DataFrame(lib.ismember(df.values.ravel(), set(items)).reshape(df.shape), df.index, df.columns)
```

_could later generalise ismember to take a 2d array if you wanted._
 `flatten` makes a copy use `ravel` instead
 Thinking more about the axis argument.  I've been thinking about row-wise mostly (axis=0).  This will return a Series of bools where the index is the same as the original DataFrame.  Presumably axis=1 will return a Series of bools whose index is the same as the columns of the Dataframe `isin` is called on.  I'd also like to use @hayd's naiive implementation.  **Should this go in the axis argument also?** Do we have something like `axis=None` to do things element-wise?

I think generalizing ismember will be worthwhile down the road.  But maybe we should get a python implementation first, and I can tackle the Cython later.
 Uhmm, I think I'm losing my mind a bit.  `axis=1` operates column-wise so with

``` python
In [9]: df
Out[9]: 
  ids ids2  vals
0   a    e     1
1   b    f     2
2   f    c     3
3   f    f     4
```

then we should get

``` python
In [15]: df.isin({'ids': ['a', 'b'], 'vals': [1, 3]}, how='any', axis=1)
Out[15]: 
0     True
1     True
2     True
3    False
dtype: bool
```

and `axis=0` returns

``` python
In [16]: df.isin({'ids': ['a', 'b'], 'vals': [1, 3]}, how='any', axis=0)
Out[16]: 
vals    True
ids     True
dtype: bool
```

Or do I have that backwards?

Edit: I'm pretty sure this is backwards.
 @TomAugspurger Just to step back a little, what is the benefit of using how (and axis) rather than just doing .any() or .all() after ?

I think for passing a dict behaviour should be something like

```
if isinstance(items, dict):
    if axis == 1:
       df = df.T
    return pd.concat(df[[k]].isin(v) for k, v in items.items())
```

_Edit this isn't quite the way to do it, for one thing the df[[k]] term is pretty hairy, probably better to use loc/iloc (maybe even have an iloc argument) :s_
 Okay, I went with Andy's suggestion and simplified things.  Two cases
- values is a flat array: will return a dataframe shaped identical to the first one. Plays nicely with `.any()` or `.all()` afterward
- values is a dict of either `{columns:[values]}` or `{rows:[values]`} returns a subset of the original dataframe with just those columns/rows in the dict.  This requires you to specify `axis=1` for columns or `axis=0`  for rows.

Two questions before the PR is ready:
- Is there a better way to handle the axis argument?  I ignore it if values is flat, and require it if values is a dict.  
- When values is a dict, the result of `df.isin()` might not be sorted the same way as the original dataframe.  I should probably sort it before returning, right?  What's the best way of doing that?

Hopefully I've got it to a clean merge.  I accidentally included one of @cpcloud's commits in there at one point and had to reset and force push.
 I think we don't need {row:[values]} so can just lose the axis argument (they can always transpose, and I think that's probably an unusual use case) that way we can drop it entirely. Also (I think) your implementation breaks with duplicate column names :s

I have added a couple of commits to this (and changed some of the behaviour... will add as alternative pr). Let me know what you think.
 Thanks.  Agreed on the axis argument.  It makes much more sense column--wise since the types will be homogenous down columns, but (probably) heterogenous across columns.
 this is how you should iterate to potentially deal with dups (this is from core/frame.py)

```
 def itertuples(self, index=True):
        """
        Iterate over rows of DataFrame as tuples, with index value
        as first element of the tuple
        """
        arrays = []
        if index:
            arrays.append(self.index)

        # use integer indexing because of possible duplicate column names
        arrays.extend(self.iloc[:, k] for k in xrange(len(self.columns)))
        return izip(*arrays)

```
 @TomAugspurger close this in favor of #4258 ?
 @TomAugspurger sorry...I realized that's on top of yours..never mind :)
