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.
Line is too long. [81/80]<br>Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.