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
update NEWS file
rough potential fcn for scalar potential, wip
better piecewise support
update news
potential: bug for orientation of x/y
is_unbounded is deprecated in 0.7.6, use is_infinite

Fixes #144.
remove extraneous semicolons from some tests
Better support for quotes in raw sympy commands in sym()

Fixes #143
Merge pull request #145 from cbm755/144fix

Fix 143 and 144
modify the readme, pointing explicitly to the "help wanted" issues
potential: return nan if jacobian matrix not symmetric
Merge pull request #148 from cbm755/scalpot

Add potential (scalar potential command)