Disconnect empty spans in the integrator.
  Made DOM walker safe against null elements.
  Made DOM writer safe against null elements.
  Removed empty spans from test case data.
  Readme update.

git-svn-id: https://plugins.svn.wordpress.org/instant-breadcrumbs/trunk@880639 b8457f37-d9ea-0310-8a92-e5e31aec5664