link to doxygen doc

This was created following the instructions at
http://benlimmer.com/2013/12/26/automatically-publish-javadoc-to-gh-pages-with-travis-ci/
and
https://github.com/ReadyTalk/swt-bling/blob/master/.utility/push-javadoc-to-gh-pages.sh

Todo: define a travis rule so that the docs are built and published
only for some commits (for example when the commit message contains a
trigger keyword).