Unit tests for the new Authentication APIs

* Added unit tests for testing AuthenticatorFactory, BasicAuthenticator, and TokenAuthenticator.
* Modified ReplicationTest according to reflect the code changes.
* Reference PR for Authentication APIs implementation : https://github.com/couchbase/couchbase-lite-java-core/pull/159 .
Merge pull request #263 from couchbase/auth_api

Unit tests for the new Authentication APIs