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
simplify nested test
import assume from assumptions
from __future__ import with_statement
assume -> assuming
add assume function
Add retract function.  Partner to assume.
add "see also" to assume functions
remove excess newlines in assume.py