Update documentation, add some API conveniences.

[jamadden]

Specifically, make nti.testing.mock available as an alias for the best mock package installed, either unittest.mock or mock, and let most matchers be directly accessed more quickly, e.g., from nti.testing import is_true .

Add more cross-ref links to the documentation.

Improve the description of mismatches in aq_inContextOf, also when Acquisition isn't installed.