Create OpenBabelConverter to convert MDAnalysis Universe/AtomGroup to OpenBabel OBMol

[lunamorrow]

The OpenBabelConverter will take an MDAnalysis AtomGroup (selected from a Universe) and correctly convert it to an OpenBabel OBMol. This will require conversion of both atom attributes and coordinates. Mapping between OpenBabel and MDAnalysis attributes has already been completed during the process of #12 and #16, which will significantly speed up the development of this class.

For more information and suggested implementation please see GSoC Project.