Remove preventDefault and mousedown workaround

As long as the mousedown event does not have preventDefault,
dragging out of an iframe works as expected.
Continue to preventDefault on touch
Suppress text selection behavior during drag

Reference: https://github.com/mbostock/d3/pull/1289
Add iframe debug html
Refine existing methods for disabling text selection
fix jshint errors
Merge branch 'iframe-drag'

* iframe-drag:
  fix jshint errors
  Refine existing methods for disabling text selection
  Add iframe debug html
  Suppress text selection behavior during drag
  Continue to preventDefault on touch
  Remove preventDefault and mousedown workaround
remove unnecessary preventDefault