Add number of skipped tests in test report

The pull request implements the improvement asked in [GRADLE-2976](http://issues.gradle.org/browse/GRADLE-2976), and goes a bit further.

It adds the number of ignored tests in the front page of the test report, but also on every package and class test report. It also adds this number as an additional column in the report tables, and markes the packages and classes that have no failure but have ignored tests in orange.

The success rate is also computed differently: number of passed tests / number of run tests instead of number of non-failed tests / number of tests.