WIP for issue #10

Supports a comma delimited set of secrets and tries decryption against each.
If we can figure out why the tests are failing then this should be good.
Update README
This isn't Fernet 2.0
Protect from Cipher errors within OpenSSL

but ensure that we're raising them when we need to.
Bump to 0.3.0
Merge branch 'key_rotation'