Added function `fdiv_q`

Returns the floor of the quotient when n is divided by d.
Added `Gamma` Class

Added basic API and function definitions
Fix minor issues in `gamma` function. Added tests
Merge branch 'master' into gamma

Conflicts:
	src/functions.h
Added API and function definitions for LowerGamma

TODO:
* is_canonical
* lowergamma
Added simplification of `lowergamma`. Added tests
Added TODO comments for lowergamma
Added UpperGamma class. Added tests.
Merge pull request #269 from sushant-hiray/gamma

Gamma Module. Merged #269