Fix InstrumentTask when having classesDirs set on Convention

For my multiproject setup I wanted to have one merged cobertura report.
Also when a TestClassX from module A covers code from module B, I want to have have the coverage beeing reported.
With my commit I evaluate the convention.coverageDirs property that holds a list of paths for my multiproject build.

build.gradle:


    coverage {
           rootProject.subprojects.each {
                coverageDirs << file("subprojects/${it.name}/build/classes/main")
            }

This would help quite a few people I think. (Issue #6 addresses the same thing)
Maybe you can think about it.

Thanks,
detlef