use python_cmd instead of _string in assumptions
Support negative assumptions

Fixes #140.
support finite assumption
fixes for SymPy 0.7.6

We *need* a canonicalization routine for assumptions: current code
on our end is a mess!
oo < positive won't be fixed in 0.7.6, mark xtest
fix ccode test 0.7.6 (there's been a change in codegen)
_return is the new ccodegen behaviour, clarify the test a bit
Merge pull request #141 from cbm755/076_changes

Changes for SymPy 0.7.6