Specify Encoding in Clover Configuration

This includes setting the Javac executable.
OK. The `executable` property should be of type `java.io.File` as it is the fully-qualified path to the compiler executable.

One more thought. We should probably change the DSL to be more specific about what these properties apply to. My idea would be something like this:

    clover {
        compiler {
            encoding = 'UTF-8'
            executable = file('/user/home/dev/java/bin/javac')
        }
    }