Base clases provide more protected methods for subclasses

[dgoiko]

I've prepared a branch with some modifications I had to include in the codebase in order to provide custom WebURLs with adittional fields (and the necesary diferent WebURLTupleBindings).

In order to achieve that, I had to introduce the following modifications:

• WorkQueues creates a WebURLTupleBinding. I added a protected constructor which accepts it as a parameter, so superclases can provide custom WebURLTupleBinding instances.
• Frontier creates the WorkQueues in the constructor. Now it has a createWorkQueues method that can be overriden by subclases in order to create custom subclases.
• Same than above for CrawlController and methods createFrontier createEmptyWebURL
• WebCrawler class logic to follow redirections and outgoing URLs is now placed inside protected functions that can be overriden.