add a props dict as an attribute to Species

this allows the user to store user-defined, and arbitrary
key-value pairs of his/her choice.

Conflicts:[RESOLVED]

	rmgpy/species.py
add props attribute in constructor as parameter and default to empty dict

this is more flexible than setting it in the constructor to an empty dict.
This way, one can also transfer a properties dict from elsewhere.

Conflicts:[resolved]

	rmgpy/species.py
add props parameter when calling constructor of species.Species

Conflicts:[RESOLVED]

	rmgpy/rmg/model.py
add cythonized props dict in species.pxd
add documentation in rmgpy/species.Species on props attribute
added test in speciesTest for props dictionary attribute of Species.

testing if props attribute is a dictionary, and if dictionary entries
are properly stored.
Merge pull request #278 from nickvandewiele/props_species_refactor

Add props dictionary attribute to the Species class for storing various user-defined properties.