Added sesame door system

This is a basic door system that allows one to add multiple doors to the main
menu by simply configuring them in the default.ini file. One can specify a door
as follows:

    [sesame]
    door = /path/to/door
    door_key = @

If you wish to override environment variables for the door, you can use Python
format string syntax. All of the [system] level configuration variables are
available and also session information via the session dictionary:

    [sesame]
    door = /path/to/door
    door_key = @
    door_env_USER = {session[handle]}
    door_env_ROOT = {datapath}
    ...