better handling of events and layers

- layers should have 'map-layer' class
- events are namespaced as '.mapbox' to make removal possibile
- added a 'is-dragging' class to the viewport while panning
quick fix for ie and js formatting

* after some panning, the native drag event got fired, we need to
prevent `dragstart` event
Merge pull request #4 from dwightjack/master

Added flexibility in markup and event namespacing