Averager

Added
- Created `median` function
- Created `mode` function

Changed
- Switched to using `*args` for all public functions, meaning `average([x, y])`
changed to `average(x, y)`

Removed
- Dropped support for Python 3.5

Some codebase cleanup.

Added
- Supported Python 3.5 and 3.6

A huge internal cleanup along with some new changes.

Added
- Supported Python 2.7

Changed
- Switched from classes to module-level functions
- Moved to Read the Docs

Fixed
- Bumped version to fix description on PyPI

Added
- Created tests and added CI