Install Modules for Postgres 9.1

In Postgres 9.1, contrib modules got moved over to the [extension system](http://www.postgresql.org/docs/9.1/static/sql-createextension.html). In order to make Texticle compatible with the newer version, we should make the rake task that installs trigram detect the version and act accordingly. It was complex enough that I felt like it deserved its own object and, in the process, I realized it was easily genericizable (by the power of my BA in Linguistics, I declare this is a word). Thoughts, anyone?

(This is a port of [this pull request](https://github.com/tenderlove/texticle/pull/68).)