event handling, to extend to manage other hash arguments

event handling, to extend to manage other hash arguments and map behaviors. 
our use case is representing state of visible tile layers in the hash.

```on/off```

adding/removing callback functions for when hash "change", or map "update". callback function manipulates hash, or takes action depending on hash.

```trigger```

after making a change to the map state, triggers a response to a map move

```setMovingMap/clearMovingMap```

prevent updates while making a series of changes to the map, by bracketing the code block with these calls