Phing task improvements

Here are some improvements/fixes on the ant task.
Unfortunately, **I can't test the task in an ant environment** as I have no project working with it. Perhaps @juliens could help :) 

It would be really great to have a feedback before merging this PR.

You can now configure the following reports:
* HTML/clover code coverage reports
* xUnit report (which was already supported)

The task has also been cleaned and a unit test has been added. 
@mageekguy Perhaps you will want to move the test somewhere else as it's actually in the ```tests/units/resources/phing```and is not executed when we use the ```--test-it```flag.