Add Python wrappers for pi and E constants
Fix a typo in a docstring
Wrap the exp() function
Convert sympy.exp properly

Test written. sympy.exp is a class, while csympy.exp is just pow(E, x). The
test makes sure that things can get converted to and from SymPy properly.
Python: add tests for constants E and pi
Merge pull request #323 from certik/python_fixes

Add Python wrappers for exp, E, pi