Fixed Issue #151: CrawlElement and TagElement should be refactored to one class

The file CandidateElementExtractor.java was refactored to make use of the CrawlElement and CrawlAttribute classes. Previously it was using TagElement/TagAttribute which was a duplication of code.
This required some slight modification to the CrawlElement/CrawlAttribute code.
We also removed the unused tests for TagElement/TagAttribute that will of course fail now.