updated dt.int2 to allow infinite degrees of freedeom; 
updated dtn.mix.R to switch to dt.int2 when all degrees of freedom are integers; 
updated convest.Rd examples.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/pi0@333 edb9625f-4e0d-4859-8d74-9fd3b1da38cb