Added DipoleMoment as unit type in rmgpy.quantity.

So we can handle dipole moments as physical quantities. Note that debye is
not built into the quantities package, so I've added it manually with the
symbol De (since D is taken for the Darcy unit).
Added several transport properties to Species class.

The properties - dipole moment, polarizability, and rotational relaxation
collision number - are needed in order to be able to load Chemkin transport
properties files.
Added functionality for loading/saving of Chemkin transport files.

The new functions in rmgpy.chemkin are loadTransportFile() and
saveTransportFile(). Generally you will load the transport file using the
transportPath parameter in loadChemkinFile(). Currently loadTransportFile()
assumes that all species in the transport file are also in the main Chemkin
file.

The comments in the transport file are discarded. Not sure if there's a
better way to handle them.
New command-line interface for mergeModels.py script.

You now separate each model's set of files to be loaded after a --model1,
--model2, --model3, etc. flag. You must give the main Chemkin file first,
then the corresponding species dictionary. Also new is the ability to give
the transport file for each model, so those can be merged as well.