Fix issue #33

- Implement Authenticator and AuthenticatorFactory
- Move the code that extracts credentials from url’s user info and the
authenticator object into AuthUtil so that it could be reused in
Replication and ChangeTracker.
- Port AuthTest from Android
- Still keep the Authorizer code as we may use some codes from it when
we implement Routher (Rest API Access) — Android and iOS are still
using Authorizer to do authentication inside its Router class.
Fix issue #100

- Add FacebookAuthTest to test authenticating by using Facebook.
- Add Facebook reister to the SyncGateway config file.
Use Couchbase Lite Unit Tests Facebook App Id
Merge pull request #104 from couchbaselabs/fix-issue-100

Fix issue 100
Manual merge of PR #105.
Merge pull request #105 from couchbaselabs/fix-issue-33

Fix issue #33