improve rhythmic slash notation for drum staves

On examination of drum scores & books, it seems that "rhythmic slash notation" more commonly uses regular noteheads as opposed to slashes, and places them directly above the staff, not floating higher.   But slash heads & floating above is more common / better for non-drum staves.  So I tweaked the code to differentiate by staff type.  Updated mtest.  Here, BTW, is an image showing the type of notation that is produced:

![drumset-rhythmic-slash](https://cloud.githubusercontent.com/assets/1799936/5194668/3ecc376c-74ce-11e4-9b44-c79e7fab2645.png)