Added support for JUnit XML output

As requested in gruntjs/grunt-contrib-jasmine/issues/11 this pull request adds support for JUnit XML output to the grunt-contrib-jasmine task. I wasn't completely clear on how you were intending to implement it but it seemed from a comment that you wanted to keep most of the logic in `PhantomRunner` so this is what I did (gathering some additional data so that we could give the duration of each test).

I've tested the produced output against our code base (currently > 250 specs) and our Jenkins install and it seems to behave more or less the same as `grunt-jasmine-runner` used to.

I used an underscore template to build the XML file rather than the string concatenation approach in `JUnitXmlReporter`. I also added some extra fields to the JUnit output (see commit message for ef8f191c46068649a7e6b626be394a8d1e37fe18 )