Added DOM diff view to CrawlOverview plugin

Added a new tab "DOM diff" to the CrawlOverview's state
page. Standard behaviour is to show diff from current state
with previously visited state, but users can select other files
to compare as well.

It is mostly client side, using some js libs and html5 localStorage
and fileReader. Works in all browsers with html5 support.

Changes to Java / templates:
- Added option to CrawlOverview to persist stripped DOM instead
  of full DOM.
- Added html and context code to state.html, header.html and
  StateWriter.java
Merge pull request #418 from ivan-p92/overview-diff-viewer

DOM diff view in CrawlOverview