Add mongoose code for embedded web server

Taken from mongoose master (667707df)
Add RESTful Helium service

Expose similarity and substructure search via REST,
thus avoiding the overhead to having to load index
each time as in the case of commandline tools.

Examples of usage:

curl 'localhost:8089/helium/similarity?q=c1ccccc1Cl'
curl 'localhost:8089/helium/substructure?q=c1cccc(cc(CCN)n2)c12&pretty=true'