Add support for adding multiple test tasks per capability/browser type to geb-saucelabs

[spartanhooah]

Currently the only way to execute groups of tests is through a hackish workaround: set a project property on the command line (e.g. -PstuffToTest=a) and then do something like this in the build.gradle file

sauceLabs {
        task {
            if (stuffToTest == "a") {
                filter {
                    includeTestsMatching 'stufftotest.*'
                }
            }
        }
    }
}

C.F. this usergroup thread: https://groups.google.com/forum/#!topic/geb-user/aytecRJo45g