Add an HTML dependency report

This is an implementation of the suggested contribution described in [the forums](http://forums.gradle.org/gradle/topics/add_an_html_dependency_report).
An example report for the gradle project itself (and all its subprojects) is browsable at [http://jnizet.github.io/gradle/](http://jnizet.github.io/gradle/). A fake `foo:bar:1.0` dependency has been added to the core module to show how unresolved dependencies look like in the report.