Work to abstract MongoDB from the MoleculeRef

The ObjectRef/MoleculeRef class only really needs to store the string
corresponding to the mongo::OID, this then removes the need to include
MongoDB headers in the MongoChem public API. Also moved other includes
that were not necessary, keeping Boost out of the Qt headers so that MOC
can successfully run with the latest version of Boost/MongoDB client.

Change-Id: I614b3a789551884abdcc114969af07cd9dec3869