game: limit to maximum of 5 enemies at one time
add license and copyright to some files
don't use gamestate.registerEvents()

implement our own callbacks and call the gamestate functions
manually from them. only implement the ones which are necessary:
currently update(), keypressed() and draw().
use cock instead of TLbind for input
game: show pause screen when escape is pressed

has menu to resume or quit to main menu.
draws game in background.
Update to love 0.9

Update libraries to newer versions.

Minor changes to conf.lua.

Replace love2d-assets-loader as a submodule with the single loader.lua
file instead. Make the minor change to this file to port it to 0.9,
since upstream hasn't yet been ported.

This also makes the require path for it much nicer: upstream's repo
layout doesn't lend itself well to being included as a submodule.