ENH: allow dataframe get to take an axis argument
closes #6703

I would have like to have a solution also working for panels, but I've never used them, so I'm not sure how it should behave.

Ok, I used your suggestion and put the tests in `tests/test_generic.py`... but it still is the same test. I should probably make more tests, in particular to test what happens for Panel.

(Sorry, Atom couldn't help but by itself remove some spaces that were at the end of their lines)
 I don't understand, why is the build failing?
 If you go to 'Details', you can get the build's output. In this case:

```
======================================================================
FAIL: test_getitem_get (pandas.tests.test_series.TestSeries)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/pydata/pandas/pandas/tests/test_series.py", line 1339, in test_getitem_get
    self.series.get(-1), self.series.get(self.series.index[-1]))
AssertionError: None != -0.2267325702945841
```
 Uh, yes, thanks.

I think I've identified what is going on, but I'm not sure on how to proceed:

```
In [56]: pd.Series([0,1], index=[0,1]).get(-1)

In [57]: pd.Series([0,1], index=['a','b']).get(-1)
Out[57]: 1
```

So, basically, the result of the `get` method for a series depends on the type of the index... Which is not the case for a dataframe:

```
In [63]: pd.DataFrame([[0,1],[2,3]], columns=['a','b']).get(-1) is None
Out[63]: True
```

I could redefine `Series.get` to supersede `NDFrame.get`... but I don't know if that's a good solution. Maybe there are other classes inheriting from `NDFrame` that work like `Series`?

Here is what I'm proposing: if the solution based on the slice and `loc` fail, and `axis` is `None`, we revert to the old implementation. Thus we maintain a backward compatibility...
 I'm not sure this is a good idea. Get is a standard method for mapping types and we already have a lot of indexing methods.
 yeh, this is getting into too many edge cases. Lets just keep `.get` simple.
