Make dragSelection changeable after init()

Adds enableDragSelection(), disableDragSelection(), toggleDragSelection()
Support for dynamically changeable 'dragSelection'

Modifies and combines the two 'mousedown' listeners for use with dragSelection.

Also adds feature to ignore the mousedown's that occur on the possibly present horizontal scrollbar (resolves issue #181)
Add documentation for new methods

Added enableDragSelection(), disableDragSelection(), toggleDragSelection()
Add missing comma

oops
Merge pull request #230 from rgagnon24/master

Add dragSelection methods, ignore scrollbar clicks