Added Settings module & 'merge' functionality

Previously, app.rb and models.rb manually read YAML options. Now, they
just call module-level methods of Settings.

There was also no way to merge configuration settings. This would be a
handy feature for 'config/site.yml' because, chances are, someone does
not want to have separate site titles for production and development.

As of this commit, you can put 'common' settings into site.yml that act
as defaults under any environment.
FIX: Settings throws exception for blank settingss
Updated example site.yml