Array contains operator ( @> ) support

1. Add support for the Postgres array contains operator. This operator
is like overlap, but expects all of the provided elements to be in the
array.

    MyModel.where.array_contains(tags: [1, 2])

2. Moved the array nodes into their own file
Update querying doc to include array_contains
Remove explicit array_contains where extension, use contains instead

Removed the array_contains where extension and changed the existing
contains extension to check the type of the column. It will use either
array_contains or the INET contains depending on whether the column is
an array or not.
Merge pull request #67 from jagregory/array_contains

Array contains operator support