Range selector is now a plugin

Conflicts:

	dygraph-combined.js
Moved check for labels existence into reparseSeries since that method is also called by Dygraph.updateOptions()
Need to call createDragInterface_() after plugins are activated in order to allow a plugin to change the default interactionModel.
Changes based on comments from pull request
More range selector auto tests
Handling zoom handle left position as a floating-point number for better accuracy at the extremes.