head(0) and tail(0) return empty DataFrames
closes #11930 

frame.head(0)  and frame.tail(0) now return empty DataFrames.

make sure to test groupby! (need updating maybe)
 Fixing the broken unit test....
 groupby is already tested: https://github.com/pydata/pandas/blob/a89f48940cd0a7346302311011beb3a09e9be292/pandas/tests/test_groupby.py#L1912-L1913.

thanks! lgtm, but please squash.
 I don't really understand why Travis is failing - it seems to be some kind of connection error?  Is that something I've done wrong?  
 Looks unrelated. squash and maybe the tests will pass next time :)
 I'm sorry, I'm not sure what you mean by 'squash'.
 http://pandas.pydata.org/pandas-docs/stable/contributing.html#combining-commits
 Ah, ok.  I think I did that right, hopefully the tests will pass this time. 
 Need more thing, a release note in `doc/source/whatsnew/v0.18.0`, maybe under the API changes section, to document the change. You can squash that commit as well (which you did correctly). Thanks!
 Added a note in the release docs.
 ok, lgtm. ping when green.
 Great!
