Fixes #1416 -- replaced assertions with error checking in EC key from numbers.

Includes tests.
Do these checks in pure python eagerly.

We get errors in different places, depending on py2k vs. py3k if we don't.
Move the supported marks to class level for fernet
Document the change in ``p`` and ``q``s lengths in the CHANGELOG
Merge pull request #1418 from alex/fix-ec-complains

Fixes #1416 -- replaced assertions with error checking in EC key from numbers.
Merge pull request #1419 from alex/cleanup-fernet

Move the supported marks to class level for fernet
Fixed an issue in fernet where the wrong exception would occur on an all-nulls input.

Also switched a few tests to not generate a key
eep, stray space
Merge pull request #1421 from alex/fix-bug

Fixed an issue in fernet where the wrong exception would occur on an all-nulls input.
Merge pull request #1420 from alex/document-pq-things

Document the change in ``p`` and ``q``s lengths in the CHANGELOG
Add MACContext and make HMAC and CMAC to use it.
Ooops.
Add docs.
Add missing period.
Remove algorithm property from MACContext and alias CMACContext.
Add to changelog. Document deprecation.