Add Player (& other entities)

add debug overlay (F3)
add very (very) rudimentary entity system
add Mob class
add Player class
add Level class
move all MainMenu buttons into one array (to allow for easier tab
selection implementation)
implement basic layer movement (using arrow keys)
add button enabled boolean
add button selectability (use tab to cycle through buttons, use enter to
'click')
remove Key inner class from Input.java, replace with booleans for each
key (bad)