added yourkit-style CSV printer and test

Hi,

in the plat_form (www.plat-forms.org) web framework comparison we use ruby-prof to profile our ruby solutions. To make it easier comparable with the other solutions, I added a printer that prints a CSV file similar to that of Yourkit. 
It uses the following columns:

Name, File location, Time (ms), Avg. Time (ms), Own Time (ms), Invocation Count, Min. Level

And prints one row for each method called together with total time, own time etc. over all invocations of that method.