Add postgres full text search feature

That's a PR related to the following topic: https://groups.google.com/forum/#!topic/rubygems-org/IR5y8bQQpcg

It's just a basic usage example, gem can be configured for more complex stuff like associations searching, results weighting etc.

I've also changed a sorting order from downloads count to search rank (which seems to be more reasonable for full text searching but can be change in a second.)

I would love to compare query times on live data but i assume it's not possible to somehow obtain those data? :)