Track Model#dirtyAttributes at write-time

[jordanstephens]

• Can we track these changes to Model attributes at write-time instead of aggregating them at Model#dirtyAttributes read-time? Maybe via mobx's observe.
• If we can track this state at write-time, we should be able to derive Model#isDirty directly from this state, rather than via a reaction where we serialize the current attributes. Consider replacing that code as well if the write-time approach is tenable. Let's try to get that serialization off of our write-path!

[jordanstephens]

This was addressed in #40