Adds a reasonably feature complete implementation of full text search
using the built in PostgreSQL engine. It uses public APIs from
Expression and Lookup.
With thanks to Tim Graham, Simon Charettes, Josh Smeaton, Mikey Ariel
and many others for their advice and review. Particular thanks also go
to the supporters of the contrib.postgres kickstarter.
These refactorings making overriding some text based lookup names on
other fields (specifically `contains`) much cleaner. It also removes a
bunch of duplication in the contrib.postgres lookups.
Matthew Somerville
Claude Paroz
Marc Tamlyn
Josh Smeaton
Thomas Chaumeny