Add evaluation of polynomials using Horner's method
Add function to evaluate even and odd polynomials
Improve documentation
Switch to generic types.

We inline it anyway so there is no reason to stick with Double
Fix calculation of initial approximation for inv. inc. beta

Initial approximation from AS64 falls out of [0,1] interval for
some inputs. Those approximation were replaced with one from "Numerical
recipes" it's esentially the same approximation but it doesn't return
values outside of range.

Second problem is crashes when derivative overflows. It becomes infinity.
`x' on next step becomes NaN and function crashes. Additional check is
added to avoid crash here. It's very rare and difficult to find. QC have
virtually no chance to stumble upon it.

Fixes #13
Better error messages
Add note about rounding
Fix invErfc w.r.t. NaN
Use new functions to compute polynomial approximation for digamma

Also next term is expansion is added. Changes to last term of the
expations makes test fail. So just to be sure I added next one.

GHC successfully inline calculation of polynomial.
Better documentation for constants

Add Euler–Mascheroni constant
Use Euler–Mascheroni constant from N.M.Constants
Describe differences between chebyshev and chebyshevBroucke
Add test for checbyshevBrouke and fix instance for Ch
Add changelog