Impacts #7 by only marking an association as changed if the value has actually changed. Removes the _attributes setter because it is kind of unrelated to whether or not the actual model is dirty. TODO: figure out if there should be a way to tell if any of the model's associatiated models are dirty.
bump version to 0.4.0
Merge pull request #9 from jmpage/non-changes-are-considered-changes

Non changes are considered changes