Add MongoChem Extension

This adds a new extension which allows avogadro to interact
with MongoChem via RPC. Now, the user can click on the "Find Similar
Molecules in MongoChem" entry in the Extensions menu and MongoChem
will automatically display the 10 most similar from its database.

Change-Id: I8633ab4cda24b73f59fa878b9854aebc2d91f175
Merge topic 'similar-molecules-rpc' into master

8abe7c63 Add MongoChem Extension