add faroo engine

I have implemented support for the faroo engine.

Furthermore I have added a function named ```searx_useragent()```, to provide an user-agent string which tell the api-server that searx is requesting something (required for faroo, and would also be nice for other engines which have an official api). It would be nice if the version-number of searx also appear in that string, but I don't know how to do that.