Rails4 support, PostgreSQL array and hstore fields support

This is an update to support Rails4 (using the modification proposed by edwardvalentini in his own fork) and the use of array fields and hstore fields as supplied by PostgreSQL.

The Rails4 modification may render yaml_db unusable with older versions of rails (this needs to be verified). The PostgreSQL support changes are compatible with other kind of database systems. That doesn't mean it supply support for array and hstore fields for other database systems...

Cheers!

Guy