Okay, I've updated the pep451 branch in the clone to include as much of the implementation as I've completed, which is the bulk of the functional changes.  It's enough to pass the test suite.

Here's what I'd like to get done before the feature freeze (in this priority order):

1. wrap up the current functional changes in the clone;
2. change module.__initializing__ to module.__spec__._initializing;
3. refactor importlib loaders to use the new Finder/Loader APIs;
4. refactor pythonrun.c to make use of __spec__;
5. implement the deprecations and removals;
6. adjust other APIs to use __spec__ (pickle, etc.);
7. add rudimentary doc additions for the new APIs.

Other things that can (but don't have to) wait until after the beta release:

* finish doc changes;
* fill in gaps in test coverage (there shouldn't be much due to Brett's mother of all test suites for importlib)

I haven't had a chance yet to make any changes to Doc/reference/import.rst in response to Brett's review, but I did make the Doc/library/importlib.rst changes he recommended.