Don't clone layer objects

In general, layers aren't safe to clone -- they may contain
references to the map, to themselves, to arbitrary other objects
which should not be cloned.

The example for which cloning was added has since been removed
anyway (6afda200, c1ee4381).
Fix add/remove cycling

In order for the mini-map layer to be added to the mini-map,
which is recreated each time onAdd is called, it needs to be
removed from the previous mini-map in onRemove.
Merge pull request #24 from jfirebaugh/no-clone

Don't clone layer objects