Implement Polygamma

[betanalpha]

Need the polygamma function in order to compute derivatives of the gamma function and its derivatives. Should be able to produce a reasonable implementation by leveraging the generic recursion relation to transform the domain to (0, 1) and expand the polygamma around 0. Bonus points for going the other way and using the asymptotic expansion for large arguments.

With a polygamma implementation we can then implement the last cmath functions,
http://www.cplusplus.com/reference/cmath/

• tgamma
• lgamma

[betanalpha]

Implemented in Commit 345f6da, quadgamma may cause problems for small arguments.