Fixes related to urls

Hi all,

@wilcovl and I have collected some small bugs while working on Crawljax, which we'd like to fix before we push before we issue pull requests for our other work.

046dcae4410c04214d6bad097777d0847456040f Fixes a broken link in the crawl-overview package.
ed946a1a83fc50ed939214dd1bcb54454d1e6083 Catches a null-pointer exception, which is thrown when the host returned by [getHost()](http://docs.oracle.com/javase/7/docs/api/java/net/URI.html#getHost()) is undefined. we were not sure what would be the preferred way of handling this error (multi-catch, seperate catch, if-check). So we just used the shortest solution.
e627c2990d9a4248d74cae143bd1b1d760cddb8a Normalizes the URL representation in the crawl-overview package by pruning the trailing slash. This will prevent display of example.com/ and example.com or example.com/blogpage and example.com/blogpage/ as unique URLs on the URL-page.