Add parameter configURL to CheckStyleTask (GRADLE-932)

This pull request adds the parameter `configURL` to Gradle's CheckStyle plugin which defines a URL (HTTP or HTTPS) from which CheckStyle will fetch its configuration (see [GRADLE-932](http://issues.gradle.org/browse/GRADLE-932)).

I'm not quite happy yet with the [fallback check for `configFile` and `configURL`](https://github.com/smarchive/gradle/blob/ed2a19bffdc900de36df4392894240ac548884e6/subprojects/code-quality/src/main/groovy/org/gradle/api/plugins/quality/Checkstyle.groovy#L168-170). Maybe you have an advice how to do this better.