- add support for "**" as notion for "all available classes" selector
- do not register -cp paths as source path; the difference is now
  that classes within sourcePath are "in scope" for doclets, while
  those in classPath are used for further lookups - but they won't
  be examined in the doclet's manner.
- when placeholders evaluate to no added classes, then do not
  try to add them - instead just skip and continue with next element.

  Only bad about this is that one might as hasNext() first, then
  calls next() which might throw an exception against expectation,
  when the next element evaluates to no classes and there is no
  following one...
Merge pull request #323 from kiesel/doclet-improved

Doclets improved