Complete input subsystem.

 [NEW] Complete input subsystem.
       nexus.input.isKeyDown
       nexus.input.isKeyUp
       nexus.input.isKeyTrigger
       nexus.input.isKeyRepeat
       For now, default repeat interval is 15, later might be configurable.
       In the love.update callback must call nexus.input.update to update the
       state table.
 [MOD] Change the function calls to new input subsystem.
 [FIX] Fixed the bug in title screen when pressing down.