ENH: Dataframe should have a .isin() method
Any reason not to give `DataFrame` a `.isin()` method like `Series`?

The new wrinkle is that the user needs to specify if they want a logical OR or AND.

e.g.

``` python
In [26]: df = pd.DataFrame({'vals': [1, 2, 3, 4], 'ids': ['a', 'b', 'f', 'f'], 
'ids2': ['e', 'f', 'c', 'f']})

In [27]: df
Out[27]: 
  ids ids2  vals
0   a    e     1
1   b    f     2
2   f    c     3
3   f    f     4

In [1]: other_ids = pd.Series(['a', 'b', 'c', 'c'])

df.isin(other_ids, how='or')

  ids ids2  vals
0   a    e     1
1   b    f     2
2   f    c     3

df.isin(other_ids, how='and')
```

See [this SO post](http://stackoverflow.com/questions/17596733/filtering-rows-from-pandas-dataframe-using-concatenated-strings/17597500#17597500) maybe.

If someone else wants to take this, feel free. Can't promise a PR any time soon, but maybe in the fall :)
