Add credential stores plugging.

* tests/test_config.py:
(TestAuthenticationConfigFile.test_unknown_password_encoding):
Test that the user get a meaningful error on typos or installation
problems.
(TestCredentialStoreRegistry): Basic tests for the registry.
(TestPlainTextCredentialStore): Test plain text credential store.

* config.py:
(AuthenticationConfig.decode_password): Go through the credential
store registry to decode the password.
(CredentialStoreRegistry): New registry for credential stores.
(CredentialStore): Abstract base class for credential stores.
(PlainTextCredentialStore): Default, plain text, credential store,
using authentication.conf file itself as storage(i.e actual
behavior).