Implemented the new DOM strippers

The idea is that you define zero or more of these strippers. They do not
have to produce a valid DOM, but they're there so you can influence how
DOMs are compared. StateVertexes are compared on the equals function of
these stripped doms so the more alike you make them, the less states
you'll have at the end of your crawl.
Refactored out filtered attributes and the old filter mechanism.

Also deleted test utils since they're not used and placed weirdly in
their own package. The only method that was used has now moved to core.
That can be refactored later.

Also fixex iFrame crawling. iFrame crawling is now option. Also updated the candidate elemen
extractor to not crawl any elements anymore when this configuration is
disabled.