ENH group attribute access for HDFStore
Summary:
- `store.set_attrs(key, **attrs)` to set attributes of a group
- `store.get_attrs(key, attrs, default)` returns namedtuple with values from a groups attributes or single attribute directly if attrs is specified as a single string
- `to_hdf`, `put` and `append` have optional `attrs=dict(...)` argument to update attributes when storing an object

Examples:

```
from pandas.io.pytables import get_attrs
df.to_hdf('h5', 'df', attrs=dict(a=1, b=2))
a,b = get_attrs('h5', 'df', ['a', 'b'])
df = read_hdf('h5', 'df')
```

```
st.put('df', df)
st.set_attrs('df', a=1, b=2)
a,b,c = st.get_attrs('df', 'a b c', default=None)
a = st.get_attrs('df', 'a')
b, = st.get_attrs('df', ['b'])
```

```
st.append_to_mutple({'grp/a': ['A1', 'A2'], 'grp/b': ['B1', 'B2']}, 'grp/a')
st.set_attrs('grp', a=1, b=2)
attrs = st.get_attrs('group', ['a', 'b'])
```

this was originally discussed in #2485. Given that I show this in the cookbook, might be a nice feature.

Couple of issues:
- you could deserialize this and store in `df.attrs` as a dict (would need to add `attrs` to `core/generic/NDFrame._metadata` so this is recognized.
- what happens if you append and specify `attrs` on multiple calls: merge the data, overwrite, raise?
 @cpcloud I know you use HDF5 a bit....what do you think about this feature?
 :+1: on this. @wabu can u give an usage example of reading attributes in the top of the PR?

-1 on `df.attrs` (for this PR anyway, +1 for metadata generally) bc then you can't really have attrs on group you can only have them for a particular frame.

in my use cases i have metadata that i don't want to store as an array and it may apply to _all_ the frames in a particular group.

one ubiquitous example is sampling rate, which may apply to multiple frames but i don't want to store as a single element series (which is what i do now).

as for appending data, seems like should overwrite if new values for attributes are provided and keep whatever's there already. similar to how `dict.update` works.
 when appending, attribute values are overwritten, but older keys stay:

```
df.to_hdf('h5file', 'df', attrs=dict(a=1, b=1), append=True)
df.to_hdf('h5file', 'df', attrs=dict(a=2, c=2), append=True)
with pd.get_store('h5file') as st:
    print(repr(st.get_storer('df').attrs))
```

```
    a := 2,
    b := 1,
    c := 2,
```

so I'll document the behavior?
Moreover as noted in #2485 I add a warning about the size limitation of the meta store.
 hmm, maybe better than putting this on the append call, how about a pair of `get_attrs` an `set_attrs`? more generic that way (could be on the append call too I guess, I just don't want to add an arg to `select` to `return_attrs`), I think kind of clunkty
 API like 

``` python
store.get_attrs('nested/group', ['a', 'b'])
store.set_attrs('nested/group', dict(a=1, b=2))
```

?
 yep, though could also have a `name` or something to handle different attr nodes. maybe better than adding in append.
 `get_attrs` and `set_attrs` sound good. I still would like to have `df.to_hdf(..., attrs=...)`, so one can store metadata easily. I don't know how to handle it for the read_hdf case. Perhaps returning the data and something like a named tuple, so one can:

```
df, attrs = read_hdf(..., attrs=['a', 'b'])
print(attrs.b)
df, (a,b) = read_hdf(..., attrs=['a', 'b'])
```

`set_attrs` could also accept kwargs, and `get_attrs` returns a namedtuple?

```
a, b = store.get_attrs('nested/group', 'a', 'b')
store.set_attrs('nested/group', a=1, b=2)
```
 @wabu i think that is a reasonable interface. You will restrict the keys to only non-space keys

e.g. `'a', 'b'` are valid but `a column with spaces` is not (but no big deal). you will have to check this on creation. the reason is that the named tuple won't work with them. alternatively you can return a dict (might be cleaner).

you are ONLY going to return the attrs IF `attrs` is specified (so the API is unchanged) for `read_hdf`.

also prob then need to add to `append/put/select/to_hdf/read_hdf` for consistency.
 Here's my impl. Still have to update docs n' stuff, but wanted to make get a first feedback. I updated the first comment to give an overview of the changes. I'm using `store.update_attrs`, so it's clear that it works like `dict.update`.

`to_hdf`, `put`, `append` and `read_hdf`, `get`, `select`, `select_columns`, `select_as_coordinates` all have optional argument `attrs`,  `append_to_multple`/`select_as_multple` raise an TypeError, as attrs agument is invalid.
 his api is getting to be way too bloated I would revert back to only having a `set_attrs/get_attrs` (which accept a store or a file). You can then have the default argument their. Anytime you have to multiply name an argument, e.g. `default_attrs` then the api needs work  Eliminate everything else.
 yes, totally with you on this. where should `get_attrs`/`set_attrs` go if they accept a file?
added this to stay consistent over all store functions, but already was not happy with the complexity when implementing it.
 I think it would be ok to directly import, e.g. `from pandas.io.pytables import get_attr,set_attr` if you want to use them directly with a filename (w/o a store). could also add to global imports at some point (though name would have to be different), or I guess as part of `to_hdf/from_hdf` (but that get's the same issue about the API bloating)
 @wabu ?
 ok, did the set_attrs, get_attrs thing, also let the attrs=dict() for the write functions (to_hdf, put, append), so one is encouraged to put meta data with the data and it did not complicate the code, but I can remove this also.
 @wabu status on this?
 @wabu status?
 closing for now. @wabu if you'd like to update, happy to reopen.
