fix for NUTCH-2455 more efficient usage of hostdb in generate

[okedoki]

Three questions/modification left open:

1. In several places we use url.getHost() in the Nutch code, in other we use url.getHost().toLower(). Why?
2. public static class ScoreHostKeyComparator extends WritableComparator should Implement Raw comparator. If you know how to do it you are welcome to do.
3. The whole Generator file is to big, it should be spread to several files. Again, if you know how to fix it in a good way, you are welcome.