Move GCM tag size/value validation farther forward -- this makes it easier by not requiring future backends to implement the same checks
Rearrange
Fixed test for earlier exceptino
Merge pull request #386 from alex/move-gcm-tag-validation

Move GCM tag size/value validation farther forward
Merge branch 'master' into privatize-me-captain

Conflicts:
	cryptography/hazmat/backends/openssl/backend.py