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!
Good catch, it's better to wrap \[\] inside parentheses. Will make another commit to fix this.
About "All of these could be combined into a single expression": I agree, but "Explicit is better than implicit", so I think that it's better this way, do you agree?