Fixed issue 3538. Subs working in .n()
Added test
In use2to3, only use WindowsError in Windows

Fixes issue 3457.
tweak trigsimp

- make sure c is in match result
- break loop if expression is unchanged
add rebuild rule
add yieldify branching strategy
add yieldify imports
Rewrite rule takes condition
docstring for rewriterule
rewriterule supports new assumptions
fix rewriterule doctest
evalf_trig handles complex arguments without raising exceptions
Merge branch 'master' of https://github.com/sympy/sympy into evalf-trig-iss3538
Merge pull request #1739 from asmeurer/WindowsError

In use2to3, only use WindowsError in Windows
compatibility.next import in unify/test_sympy.py
Merge pull request #1752 from mrocklin/unify-next-test

compatibility.next import in unify/test_sympy.py
assume context manager
test nested context managers
Remove unnecessary global declaration
assume restores old context after exception