Unregister for feature events when removing features
Merge pull request #1262 from tschaub/unlisten

Unregister for feature events when removing features.
Draw interaction
Add point at head of drawing
Finish lines by clicking last point
Snap to finish with configurable tolerance
Pop off last point from linestring
Ensure drawing is not finished prematurely
Updated sketch style
Stop dblclick propagation while editing
Add method to abort drawing
Allow user to set drawing mode
Add click tolerance to allow dragging while drawing
Updated style for draw example
Remove setMode method from interaction in favor of add/removeInteraction

This makes the example a bit more awkward, but eventually adding and removing interactions will be the job of an editing control.