Implement Single Log Out (discussion)

This is more for discussion than for actual merging, unless of course, it happens to be good enough to merge.

I'm making this pull request to discuss the best way to implement Single Log Out.  I went ahead and implemented it here, using a temporary map file (maps CAS tickets to php session IDs).

Some questions:
1. Is a temporary file the best way to handle this?  I could see it getting really large if sessions time out.
2. Are there any security concerns for implementing it this way?
3. File structure, class names, etc.
4. Will this work for most projects?

@kabel @saltybeagle 
This method of destroying a session with a given session_id will only work if the session has not been started yet OR has been committed. Setting the session_id after the session has been started has no effect. Also, after the session has been destroyed, the API should probably return and have the request exit as the SLO request should not allow any other session to be created.