add monitor code to monitor a consumergroup from outside

Care to take a look,@wvanbergen @eapache?

This adds the ability to monitor progress of a consumer group from outside the application doing the actual consuming.
As you'll have noted by the test failures, you can't have multiple "main" functions in a package. Example code should be done in `Example*` functions in the `*_test.go` files: http://golang.org/pkg/testing/#hdr-Examples