Add findSimilarMolecules() RPC method

This adds a new RPC API method which allows external applications
to instruct MongoChem to query for similar molecules in the database.

The function accepts an identifier and a format. Currently supported
formats are InChI and SMILES.

Change-Id: I723a0a7ca7626040dbc173252a416ea0c0cdeb1d
Merge topic 'similar-molecules-rpc' into master

0054d0fe Add findSimilarMolecules() RPC method