CAS-1238 Credential refactoring.

1. Rename Credentials -> Credential for clarity w/r/t MFA.
2. Move Credential up to authentication package since it is a core
   authentication component.
3. Add Credential#getId() so that credentials that are safe for long-term
   storage can simply implement both Credential and CredentialMetaData and
   consequent be stored directly in HandlerResult.
CAS-1238 Revert PasswordCredential to UsernamePasswordCredential.
Merge pull request #268 from serac/credentials

Credential refactoring, Credentials -> Credential, for code clarity and adherence to vocabulary of NIST 800-63.