Limit HttpAuthentication to staff's backend
Make LDAPAuthentication abtract

This is necessary so we can split Staff/User authentication.
Allow LDAP auth to be used for clients too

(For when we get there)
Fixup user searches for LDAP
Merge pull request #4 from protich/feature/authentication-backends

Authentication Backends Revisited

Reviewed-By: Jared Hancock <jared@osticket.com>