More features and refactoring

[glennmatthews]

Fixes #6, I think.

In brief - you can now optionally specify a Diff subclass when calling diff_from, diff_to, sync_from, or sync_to. The main reason for doing this is that after creating and populating a Diff with all relevant data, we now call a complete() callback on the Diff object; by default this does nothing, but a subclass could, for example, respond to this callback by printing the completed diff, saving it to a file, or writing it to a database.

[glennmatthews]

Additionally:

• Add continue_on_failure option to sync_to/sync_from (Configurable abort-on-failure vs continue-on-failure #8)
• Move the CRUD APIs from the DSync class to the DSyncModel class. This is a pretty substantial refactor but I think it puts us in a better position to move forward, as now we can have a small amount of CRUD logic per DSyncModel instead of a tremendous amount of CRUD logic in a single DSync class.
  • Just realized I haven't updated the README for this change - will do that soon.

[dgarros]

Looks good overall, I'll try to integrate that with NI tomorrow to see how it's working with all these changes

[glennmatthews]

Sorry, one more. I started working on #9 and realized that it would require adding another boolean parameter to the sync and diff APIs, and decided to revise these APIs to take a single flags parameter rather than having a whole bunch of individual boolean args.

This latest update fixes #9 by adding two more flags, SKIP_UNMATCHED_SRC and SKIP_UNMATCHED_DST. I'm totally open to suggestions for renaming these - could also be something like NO_CREATE and NO_DELETE, or SKIP_MISSING_DST and SKIP_MISSING_SRC, or ...

I also added a dsync field on all DSyncModel classes, which can be set when calling DSyncModel.create() or will be set automatically when calling DSync.add(). This way we don't have to re-pass the DSync as a parameter when calling the DSyncModel update() and delete() APIs, and generally makes them a bit more intuitive, I think.

[dgarros]

The flags looks good, not sure about the names as well so let's go with that for now.
The other changes looks great too