Add fields for VARCHAR and INT arrays

Add ARRAY support for VARCHAR and INT types.
Define array of TEXT instead of VARCHAR

Text is the default character type in PostgreSQL. So, using it helps in
reducing the necessary CAST operations.

In any case, the types for PostgreSQL are the same, with the only
exception of not allowing to have length limits on the elements of the
array.
Support for @>, <@ and && ARRAY operators

Add support for the operations "contains", "is contained by" and
overlap" of arrays.
Add docs for array fields

Add docs for how to use an ArrayField with South and how to use an index
on the column.