Unittest

adds a basic unit testing framework:
[x] conversion tests: to and from both SymPy and Python
[x] parser tests
<del> [x] web frontend tests</del> (not yet working on Travis)
[x] command line frontend tests
<del> [ ] test building the documentation?</del> documentation tests added to travis with 28eec10b.
[ ] test pattern matching?

Also some minor fixes to converting functions and expression (found when writing the tests)

store `__version__` in `mathics/version.py`