Handle removeLayers([]) where some elements aren't in the MarkerClusterGroup
Update Build
Fix up some bugs that happen when removing and adding a MarkerClusterGroup from the map.

Was leaving layers in _layers which meant they'd get added again when we were re-added. Wasn't unbinding our events from almost anything.
Fixes #94
Update build
Add options.spiderfyDistanceMultiplier, allows making spiderfied markers go futher away. Fixes #100
Document spiderfyDistanceMultiplier
Update build
Add eachChildLayer, an eachLayer implementation. We cannot override eachLayer as we have different behaviour. Doing so makes us add all makers to the map always which is dumb :(

Fixes #102
Update build
Redo eachChildLayer, turns out with a bit of love we can override eachLayer, yay :)

Refs #102
Update changelog and build
Fix clearLayers not unsetting __parent of the markers, preventing them from being re-added. Fixes #99
Update build and changelog
Fix map.removeLayers(MCG) not doing anything. Thanks Driklyn. Fixes #108
Update build
Update README.md

Swap showCoverageOnHover/zoomToBoundsOnClick Fixes #113, thanks HolgerJeromin!
Add a test case for #114
Improve test case
Fix #114 bug 1.
Make us not crash if we have a spidified cluster and it has a marker added to it. Fixes #114 bug 2