polys: implement modular GCD algorithm for univariate integer polynomials
polys: add tests for modgcd_univariate()
polys: implement modular GCD algorithm for bivariate integer polynomials
polys: add tests for modgcd_bivariate()
polys: implement modular GCD algorithm for multivariate integer polynomials
polys: add tests for modgcd_bivariate()
Merge branch 'master' into modgcd

Conflicts:
	sympy/polys/polyerrors.py
polys: change docstirings to be 'raw' strings in modulargcd
polys: rewrite _swap function in modulargcd

No more conversions to dense representation are used for this function.
polys: some minor changes in modulargcd

In modgcd_multivariate: 
The computation of the bounds is now faster and more readable.

In _modgcd_multivariate_p:
Checking if the result of the recursive call is a constant
is now done with `ha.is_ground` instead of `ha.LM == zero`.
polys: choose evaluation points at random in _modgcd_multivariate_p

and add a test for this
polys: rewrite _gf_gcd function in modulargcd

No more conversions to dense representation are used for this function.
polys: speed improvements in modulargcd

In _modgcd_multivariate_p unlucky evaluation points are now detected
earlier.
polys: remove examples in the docstring of _modgcd_multivariate_p
Merge branch 'master' into modgcd
polys: fix problems with python 3 in modulargcd
polys: add literature for modulargcd
polys: add more tests for modulargcd
polys: remove unnecessary while loops in modulargcd
polys: add documentation for modulargcd to sphinx