CAS-1238 Port handlers to new AuthenticationHandler interface.

1. Port all existing AuthenticationHandler components to new AH interface.
   This afforded some opportunity for relatively minor refactoring.
2. Move CAS 3.0 AH interface, related 3.0 components, and
   LegacyAuthenticationHandlerAdapter to legacy module to provide
   transition path for third-party authentication handlers.
3. Remove CAS 2.x transitional components from legacy module.
4. Remove deprecated authentication-related components.
CAS-1238 Fix javadoc typo in JAAS handler and improve formatting.
CAS-1238 Introduce local variable for clarity per code review feedback.
CAS-1238 Use commons-io IOUtils to close reader.
CAS-1238 Fix variable name typo.
CAS-1238 Provide message in exception thrown on authn failure.
CAS-1238 LegacyAuthenticationHandlerAdapter unit tests.
Merge pull request #270 from serac/port-handlers

Port authentication handlers to new AuthenticationHandler interface.