Array contains operator support

Hello,

I've added support for the Postgres array contains operator `@>` (see: [Array Functions and Operators](http://www.postgresql.org/docs/9.1/static/functions-array.html)). I followed the existing patterns to implement this, tests included.

The only thing I'm not entirely happy about is that the ActiveRecord query extension is called `array_contains` because `contains` was already taken by the INET feature. Having `array_contains` and `overlap` seems inconsistent, but perhaps that's something we could address later.

If you're happy with this change and how I've done it, I'll do the same for the array is contained by operator `<@`.

Cheers,
James