Fixed issue 3538. Subs working in .n()
Added test
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
assume context manager
test nested context managers
Remove unnecessary global declaration
assume restores old context after exception
simplify nested test
import assume from assumptions
from __future__ import with_statement
assume -> assuming