Adding support for cross-building against 2.11.

* Fix/document tests.
  - Add difflib to run diffs by hand
  - Check changes to see if it's related to directory layout of examples vs. generated
  - Attempt to display nicer diffs on error.
* Add cross-source support for 2.10->2.11 and even binary-version directories.
* Make sure 2.10 tests pass.

2.11 tests still broken, cc @retronym / @gkossakowski