new function for dispersion tests. currently very limited

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/countreg@63 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
small problem with disptest.Rd

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/countreg@64 edb9625f-4e0d-4859-8d74-9fd3b1da38cb