Defined association methods on Fog::Model to dry up models.

I've refactored the models by creating association methods. There is no need to define an attribute and a method to get the associated model anymore. Just call `has_one` or `has_many` to create the attribute with an alias and the methods to get the association.
Use the new Ruby 1.9 hash syntax.<br>Put one space between the method name and the first argument.