polys: implement the modular GCD algorithm for polynomials over
algebraic fields
polys: add tests for func_field_modgcd
polys: some improvements in modulargcd

_trunc now uses PolyElement.rem() directly.
_minpoly_from_dense now uses DMP.terms() directly.
polys:  improve code style in modulargcd
polys: add documentation in modulargcd
polys: additional tests for modulargcd
polys: add documentation for modulargcd to sphinx
n*m is even if n+m is odd (old assumptions)
n*m is even if n+m is odd (new assumptions)
UnXFAIL Wester test_H3
Fix a typo in the cheatsheet.
Merge branch 'master' into modgcd_funcfield

Conflicts:
	doc/src/modules/polys/internals.rst
	sympy/polys/tests/test_modulargcd.py
Run Sage tests in Travis
Specify a dummy Python version for TEST_SAGE in .travis.yml
Use sudo with apt-get
Merge pull request #2417 from asmeurer/travis-sage

Run Sage tests in Travis
Merge pull request #2314 from skirpichev/unxfail-wester-H3

UnXFAIL Wester test H3
A couple small fixups.
Use \, to separate the integration variable.
Whitespace fixes.