CAS-1219 Check TGT validity at start of login flow.

Replace boolean ticket check state with a tristate action that checks the
flow request context and ticket registry for TGT validity and returns
one of the following events:
1. notExists
2. invalid
3. valid
In the second case the browser cookie is deleted and the ticket is deleted
from the registry (if found).
Merge branch 'master' of github.com:Jasig/cas into CAS-1219
CAS-1219 Factor out SSO end actions into separate component.

The actions required to end the CAS SSO session have been factored out into
a common component, TerminateSessionAction, that is called from both login
and logout flows at the appropriate time.
CAS-1219 Javadoc improvement recommended in code review.
Merge pull request #277 from serac/CAS-1219

CAS-1219 Perform TGT Validity Check at Start of Login Flow