Read/write attribute directly from @attributes to match ActiveModel

[kainosnoema]

Resolves #113

This allows overridden getters/setters to be written like ActiveModel getters/setters and call read_attribute and write_attribute to get the raw value from the attributes hash. This previous behavior was unexpected to users bringing existing code from ActiveModel.

The other benefit of this change is that ActiveModel::Dirty can be included and will actually behave properly since it overrides write_attribute, expecting it to be called when a setter is called.

While I feel strongly that this should be fixed, it's a very significant change and should probably accompany a decent version bump. Thoughts?