multiple current tests and meaningful module name

Hi,

We ran across this issue when we tried to use qunit-reporter-junit with phantomjs and asynchronous tests.
It seems there is a legal case where the testStart callback is called for several tests in a row although although the prior one hasn't ended yet so I basically extended the currentTest variable to be a dictionary of current tests. This solved the problem for us.
I also implemented a method that tries to provide a meaningful modul name in case the test doesn't provide one, which makes it better for some cases and at least not worse for others.

regards

Reinhold