Get rid of unnecessary cast
Session improvements

Basic changes:

 - add isActive() to check if a session is already active or not
 - update getID() to work before init() has been called
 - get exists() to return false if not active, preventing the creation
   of a new session purely to test if something exists.
Merge branch 'master' of github.com:ufront/ufront-mvc

Conflicts:
	src/ufront/web/session/FileSession.hx