Merge pull request #160 from MathCookie17/master (v2.0.0)

A summary of changes from v1.4.2:
* Added a parameter to lambertw allowing you to calculate the non-principal branch
* Changed all cases where a constant or parameter is returned directly, to reduce the likelihood of accidental mutation
* Changed how tetration for bases <= e^1/e works: now the property x^^(n + 1) == x^(x^^n) always holds, and small bases with large payloads behave correctly.
* Fixed inaccuracies in f_gamma
* Decimal.dNegInf is now {-1, Infinity, Infinity} instead of {-1, -Infinity, -Infinity}. This one is a breaking change, hence the version number is now v2.0.0.