Lots of stuff to make it more PGXN-ready

Also should support earlier versions of PostgreSQL, though I don’t have any handy to test (might get to it later).

Things I did not do but still recommend:

* Add an uninstall script for < 9.1.
* Document the new functions and operators
* Add indexing support
* Add inverted index support if you can (GIN and GiST). See the [hstore code](http://git.postgresql.org/gitweb/?p=postgresql.git;a=tree;f=contrib/hstore;hb=HEAD) for a nice parallel.

Thanks!

David