IGNORE and SKIP_CHILDREN_ON_DELETE model flags

[glennmatthews]

Fixes #13.

• Add new DSyncModelFlags to DSyncModel which can be set per-class or per-instance as desired.
  • DSyncModelFlags.IGNORE causes an object and its children to be omitted entirely from any diff calculation
  • DSyncModelFlags.SKIP_CHILDREN_ON_DELETE causes DSync to not call .delete() on children of a deleted parent object -- they are still remove() from the DSync local data itself, however.
• Hard-code enum bitflag values instead of using enum.auto() just to make serialization/deserialization a bit more future-proof.
• Moved most of the diff calculation logic from DSync to a new DSyncDiffer class in order to reduce the amount of shared state we need to keep passing around during recursive diff construction.
• Refactored test_dsync.py to have more granular test cases after Jake correctly gave me grief on this point. :-)

[dgarros]

Looks great as far as I can tell, there is a lot of changes so hopefully I didn't miss anything but looks exactly as we discussed

[glennmatthews]

Yeah, I should have perhaps saved the Differ refactoring for a separate pull request. Apologies.