Fix to performance problem

This fixes a problem where initialising a map with many markers (e.g.
100 or more) was slow. The slowness was caused by the fact that
addNewMarker called replot for each point. This fix causes the map only
to replot after all points have been added.
Now unbinds map click correctly

Update to performance issue fix
Merge pull request #3 from jamesdhutton/master

Fix to performance problem