Dfply

- Hotfix for parsing left_on and right_on

TODO: Need to figure out fix to the inversion of symbol issue. Somewhat complicated.

Various PRs added fixing bugs, etc.

This update is almost solely the pull requests by bleearmstrong that were sitting
in the repo waiting. There were some minor bug-fixes and changes too.

- `select_if` and `drop_if` are now available to perform selection according to a function
- `mutate_if` allows variable creation if a criterion function is evaluated as True
- `row_number` window function is available (same as rank(method='first'))
- `distinct` can take no arguments, making it equivalent to `drop_duplicates` with no arguments

Lots and lots of big changes here. Total reworking of the internal functionality.
The good news is that it should (basically) work the same as before, but
hopefully better. Obviously keep in mind that this is still beta and there will
be plenty of bugs to work out on the horizon, but the preexisting tests pass for now...
There is not backward compatibility with old versions as the decorator names
have changed, but again, the functionality is otherwise the same.

Some major things (see readme for details):
- Moved entirely to python 3 support only. It may still work in python 2, but don't count on it.
- pandas-ply is no longer required; It was brittle and so rolled my own stuff.
- Selection "subfunctions" are now working and the selection functions have been changed in light of this.

- Bug fixed in semi-join and anti-join
- top_n added

Inplace piping added using the `>>=` operator. The `pipe` decorator internals
have been changed to make this possible through the addition of an `__rshift__`
implementation and chaining pipes together until evaluated against a
DataFrame.