Adds some unit tests for the solvation module.

This includes tests for: setUp, obtaining solute parameters from a library,
calculating and setting the McGowan volume for a solute, calculating solute
diffusivity, and calculating solvent viscosity.

The main method was also edited so that the unittest can be run on its own.

Along the way fixed a floating point error in calculating diffusivity.
Solvation module raises a DatabaseError when solvent is
not found in library. Unittests updated with a solvent library
test.

Conflicts:
	rmgpy/data/solvation.py
Cleaned up tests for Abraham parameter estimation and solvation
correction calculation.

Nitrogen and sulfur cases commented out for now, fixing is work in progress. Removed many test cases, so only have a few for each test.
More consistent naming of libraries in solvation module.

Used SolvationDatabase.libraries to store the solute and
solvent libraries rather than giving them their own variables,
to maintain consistency with the other types of databases. Changed
unit test to reflect the change.
Merge pull request #243 from bslakman/master

Solvation unit tests looks good to me. Tests all seem to pass.
Notice that:
    SolvationDatabase.solventLibrary
    SolvationDatabase.soluteLibrary
have been replaced with:
    SolvationDatabase.libraries['solvent']
    SolvationDatabase.libraries['solute']
which makes things more consistent with other types of database.