Merge pull request #155 from haraldpdl/superglobals

superglobals - sessions
Merge pull request #156 from haraldpdl/superglobals

replace tep_session_is_registered() with isset()
Merge pull request #157 from haraldpdl/superglobals

replace tep_session_unregister() with unset()
replace global $sessiontoken scope with $_SESSION
replace global $cart scope with $_SESSION
remove $_SESSION from global