DOC/RLS: remove duplicate content in release.rst and move remaining to whatsnew

Related to discussion on mailing list https://mail.python.org/pipermail/pandas-dev/2014-May/000272.html

So I gave it a try, moved all content of release.rst to the whatsnew file (main chunk were the bug fixes, and for the rest some api changes and enhancements that were not listed in whatsnew). All content that was in release is not in whatsnew.

Can you see what you think of this?