Merge pull request #334 from exarkun/misc-simple-extras-with-optionals

Misc simple extras with optionals
add hmac_supported method to backend.

Previously we were implicitly assuming that if a hash was supported then
its hmac equivalent was as well.
fix erroneous indent and add abstractmethod to HMACBackend
Run py.test with catpure=no for more faster.
Merge pull request #338 from reaperhulk/hmac-supported-check

Add hmac_supported method to backend
Merge pull request #339 from dreid/pytest-s

Run py.test with capture=no for more faster.
add more skip check lambdas
Merge pull request #340 from reaperhulk/skip-checks-all-around

Add more skip check lambdas
Document our API stability policy. Fixes #312
Forgotten file
Note abotu development versions
Typo
Rephrase
Merge pull request #341 from alex/backwards-compat

Document our API stability policy. Fixes #312
Slightly reorganize and cleanuip the backend docs
Cover a missed branch
Merge pull request #342 from alex/reorganize-backend-docs

Slightly reorganize and cleanuip the backend docs
Merge pull request #343 from alex/fix-coverage

Cover a missed branch
add test marks for various backend functionality
whoops, python 2.6 compatible format string