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!
Not really. In fact, doesn't this directly mirror all of the given types that Postgres allows? If so, it would be much easier to test with:

```php
if (substr($type, -2) === '[]') { /* it's an array type */ }