Delete commands destroy context

[maeneak]

delete commands destroy the gl context causing failure when removing and adding layers to re-create the source.

[maeneak]

Also added line to remove 'move' hook #16

[gampleman]

I'm not sure this is the right way to handle this. In theory the map and gl references are no longer valid after onRemove (and potentially should be allowed to be garbage collected). These references should be re-added when onAdd gets called again.

So if this is causing errors, the bug is likely somewhere else (i.e. code assuming gl context is running after onRemove has been called).