HMAC support

Conflicts:
	docs/primitives/index.rst
	tests/hazmat/primitives/utils.py
cleanup context after finalizing
remove dangling todo
address initial review comments
clean up loader and make docs default to hmac sha256
make hmac (mostly) compatible with stdlib hmac
change type of exception raised, fix docs typo
fix hmac docs to point to new hazmat location
fix indentation error and wrapping in docs
update hmac docs
Merge pull request #157 from reaperhulk/hmac

HMAC support