WIP
Updated model to have real and positive constants.

- All constants are now specified with the real=True assumptions.
- Most constants now have the positive=True assumption.
- General partial derivatives of the equations of motion should compute now.
- This only runs on latest sympy development version: 3ec66f0d7b.
- The constants yaml file now has more detail comments.
- Incomplete funtions have been removed and noused complete functions have been
  moved to utils.py.
- There is no longer a mechanism to save the matrices to disk with srepr
  because it doesn't currently support assumptions (it was also almost as slow
  as rederiving the equations of motion).
- Tests now run with nosetests.
- Added some dependencies to the setup.py.