Events draw:editstart and draw:editstop added

Two additional events are being fired upon activation/deactivation of the
edit mode, similarly to `draw:drawstart` and `draw:drawstop`. I.e. when a
user clicks on the edit or remove button, `draw:editstart` is being fired,
and when the user presses "save", `draw:editstop` is being fired.

With this it becomes possible to e.g. turn off a layer during editing and
switch it back on when editing is finished.
Documentation update and fixed variable name

Updated README.md documentation to contain the new signals. Fixed
the name of the signal property from LayerType to handler.
Merge pull request #175 from bhell/additional-draw-events

Events draw:editstart and draw:editstop added