Filter content of fetched URIs

While at it, rewrite the URI fetch logic. This gets rid of a Singleton
pattern misinterpretation and uses interfaces they way they are
supposed to be used. The code now passes the unit tests as well.

HighLevelSimpleClientInterface has been removed and replaced by the
FreenetURIFetcher interface, implemented by
InteractiveHLSCFreenetURIFetcher and proxied by NodeFreenetInterface.
Enable more compiler warnings
Allow CHK keys, show error message and let user retry with FProxy
Merge pull request #2 from bertm/basic-contentfilter

Basic contentfilter usage