Add Postgres array operators `<@`, `@>`, `&&`.

I couldn't find support for several important [Postgres array functions](http://www.postgresql.org/docs/9.3/static/functions-array.html) that are often necessary when using `ARRAY`-type columns.

Adds several methods to column objects:

 * `contains` for operator `@>`
 * `containedBy` for operator `@<`
 * `overlap` for operator `&&`

I've also added tests for these to the Postgres environment. Not really a big change, but avoids having to hack this in by hand when using the Sql library.