Support PostgreSql Array type

PostgreSql's Array type is there (http://www.postgresql.org/docs/9.3/static/arrays.html) since version **9.0**.
Now we support it in Phinx!
@shadowhand : Thank you for your comment. But this is in my mind while I define those constants: "Explicit is better than implicit. Simple is better than complex".
That's more readable this way.