[Checkers] Javadoc for Checker Framework distribution

[Michael Ernst]

I would like the Checker Framework distribution to include Javadoc, in two
places.

 1. In jsr308-checkers.zip.  It increases the size of the distribution a
    little bit, but it's too important to force people to struggle to
    regenerate.
 2. On the web, under http://groups.csail.mit.edu/pag/jsr308/releases/ or
    some other appropriate location.  Copy the Javadoc that location each
    time a release is made.  (Don't bother to archive the old Javadoc,
    since they are generated files and we don't want people referring to
    old Javadoc anyway.)

Once #2 is done, we need to update the Checker Framework manual, so that
the documentation links to the web version of the Javadoc where
appropriate.

                    -Mike