Add #all_associations method

`associations[association]` returns what is in the associations hash on the class.
When nothing is defined it will return nil, even for a collection of associations. When this happen it will fallback to the method which will return `[]` properly.

This behavior is covered on tests [here](https://github.com/fog/fog-core/pull/69/files#diff-9c020190f8605aecd293d06bf6302940R384) and [here](https://github.com/fog/fog-core/pull/69/files#diff-9c020190f8605aecd293d06bf6302940R386) for both kinds of association (`has_many` and `has_many_identities`)