Fixed parser for CountableMixtureBranchRates model.

git-svn-id: http://beast-mcmc.googlecode.com/svn@6541 ca793f91-a31e-0410-b540-2769d408b6a1
Final update for release of v1.8.1

git-svn-id: http://beast-mcmc.googlecode.com/svn@6542 ca793f91-a31e-0410-b540-2769d408b6a1
git-svn-id: http://beast-mcmc.googlecode.com/svn@6543 ca793f91-a31e-0410-b540-2769d408b6a1