Update distinct() and repartition() definitions

[zpencerq]

Update repartition functions to allow for Col in numPartitions parameter.

Reference

numPartitions – can be an int to specify the target number of partitions or a Column.
    If it is a Column, it will be used as the first partitioning column.
    If not specified, the default number of partitions is used.

Also add stub for DataFrame#distinct()

[zero323]

Thanks for the PR @zpencerq!

I think that overloading definitions would be more appropriate than Union. Logically these are two different signatures, and the meaning of the Column argument is quite different from a numeric one.

[zpencerq]

Ah, good point. I updated it in 18bced3 to reflect that. 👍

[zero323]

I guess repartitionByRange should be overloaded as well, shouldn't it?

[zpencerq]

Might as well take care of that while we're at it. 5b1b526

[zero323]

Looks good, merging into master.

Thanks for your work @zpencerq!