Source / target states in CrawlOverview

Hi @amesbah, @alexnederlof,

I implemented the changes I proposed in [my recent comment](https://github.com/crawljax/crawljax/pull/418#issuecomment-43187450) in #418. See the commit message for details.

I updated [my demo](http://iplantevin.net/ct/co3/index.html) so that you can see how it works now. I know, the demo's page names are confusing ;-) page1.html=index, page2.html=state1, page3.html=state2.

-- Ivan