Keys

this pull request should give @didory123 the majority of what he's requesting for key functionality

### To Make a New Key Named "foo"
you simply save its image to src/images/keys/foo.png

### To Add a Key Named "foo" to a map
to add it to the tmx file you just make a new object in the nodes layer with type ="key" name = "foo"

### To require a Key Named "foo" for a Door or Chest(i.e. spawn node)
set the door or spawn node's tmx property of "key" to the value "foo" 

Note: a key with the name "master" would open all locks