Added support for parsing multi-dimensional array string

* Used the Sequel::Postgres::PGArray::Parser class from
  https://github.com/jeremyevans/sequel/blob/master/lib/sequel/extensions/pg_array.rb
* The saner method to parse multi-dimensional array-strings
  is to NOT use regexps and use some sort of parser instead
* Changed String.valid_postgres_array? to use constants instead
  of variables to defined the regexps
* Converting timestamps to current Rails timezone using `in_time_zone`