Provide generic chaining principal resolver.

The new component, ChainingPrincipalResolver, supersedes the LDAP-specific
CredentialsToLdapAttributePrincipalResolver.
Remove redundant size check per code review.
Add ChainingPrincipalResolver javadocs and test coverage.
Merge pull request #301 from serac/chaining-resolver

Replace CredentialsToLdapAttributePrincipalResolver with Generic ChainingPrincipalResolver