Add drop_duplicates' subset kwarg for parity with pandas

[cowlicks]

Adding this makes the Distinct expression in blaze work easily. This came up while working on adding dask.dataframe to blaze in this PR. This should be pretty easy.

Relevent blaze code:
https://github.com/blaze/blaze/blob/master/blaze/compute/pandas.py#L216

Relevant dask code:
https://github.com/blaze/dask/blob/master/dask/dataframe/core.py#L244

Pandas API reference:
http://pandas.pydata.org/pandas-docs/version/0.17.0/generated/pandas.DataFrame.drop_duplicates.html

[mrocklin]

Agreed that this should be pretty easy. I think you can probably just pass through the argument directly to the per-partition functions.

[mrocklin]

I'll do this in about an hour if no one beats me too it first.