Added three additional masks to the data masks.

These masks are for the following features, top, bot and mid.
-top will take the top x% of the specific column and return the
entire dataframe
-bot will take the bottom x% of the specific column in the dataframe,
returns the entire dataframe
-mid returns the middle, that is the excluding the tails of the
dataset

Uses the numpy percentile package so won't work if not installed
Bumped version number
Changed .txt to .rst in setup
Updated MANIFST for pypi packaging