Bootstrap update

Arpk.start_app() is now called in the bootstrapping script.  This relies
on the new environment variables "key_bindings" and "mouse_bindings,"
which indicate the JSON files used for the keyboard and mouse inputs,
respectively.  When this method is called, the constant Arpk::Window
will be created and initialized.

Allows Arpk to report whether or not it is "headless;" this
functionality prevents calling close() on the App if no window reference
has been created.