Add postgresql-locale variable and pass it to initdb

On Vagrant, rails can't create a postgresql database because postgres gets installed with the Latin1 encoding and rails wants to create db's with unicode encoding. I added a flag to pass the locale into the initdb command,
and added a configuration option in `rubber-postgresql.yml` to make it easy to change the locale if desired. 
Tested with an install on vagrant (hashicorp/precise64 and hashicorp/precise32), and EC2 (ubuntu 12.04.4).
The default, `en_US.UTF-8`, ought to be suitable for any rails app on any US-based platform.