Grace Notes

- Adds a `GraceNoteGroup` modifier which inherits from `Modifier`
- Adds a `GraceNote`, which inherits from `StaveNote`, can be slashed for **[acciaccatura notation](http://en.wikipedia.org/wiki/File:Acciaccatura_notation.png)** by inlcuding a boolean `slash` flag in the note creation configuration object.
- Modified `Accidental.setNote()` and `Dot.setNote()` to change size if being attached to a `GraceNote`. Do we want to be more explicit than this? Maybe have an actual method to toggle the smaller grace note accidental/dot size?
- Adds a `partial_beam_length` to `Beam.render_options`
- Includes tests

![image](https://f.cloud.github.com/assets/1631625/2441326/fd30d182-ae12-11e3-89c9-3a4584657c7e.png)