Dfply

Restructuring of package to include `diamonds.csv` with pip installation
and require `six` and `pandas-ply` rather than come pre-packaged with them.

Added window functions:
`dense_rank`
`min_rank`
`cumsum`
`cummean`
`cummax`
`cummin`
`cumprod`

dplyr set operations added thanks to bleearmstrong.

`df >> union(other)`
Rows that appear in either `df` or `other`.

`df >> intersect(other)`
Rows that appear in both `df` and `other`

`df >> set_diff(other)`
Rows that appear in `df` but not `other`.