Implement dirty checking and only PATCHing modified fields

[wms]

Implements the following instance methods and properties:

• $original - cached state when initially retrieved from data source
• $dirty() - returns a boolean, true if instance has modified fields
• $pristine() - returns a boolean, true if instance has no modified fields
• $modified() - returns an object listing fields that have been modified with their new values
• $revert() - resets current state to $original

PATCH payloads now only contain modified fields. On a successful PATCH, $original is updated to the new state, making the instance $pristine() again. POSTs of new instances are unaffected.

Todo:

• Make dirty checking configurable via provider as per Only PATCH modified fields #8
• Make deep dirty checks and diffs.

[nateabele]

Hm, there's no way to hide $original inside a closure?