Flag stem placement

Here's the initial pull request for the stem/flag changes. Here's what it looks like:

![image](https://f.cloud.github.com/assets/1631625/2489129/959f8e4c-b154-11e3-9b83-3d2dff0422cf.png)

I'm thinking we should completely remove `StemmableNote.stem_extension` and have the glyph contain the full extensions (currently I implemented the new glyph properties as adjustments to the `stem_extension`). It would just require a refactor to make sure hard coded instances of `stem_extension` are removed. This would also allow us to remove setting the `stem_extensions` in `StaveNote.init()`

Also note that I have another pull request incoming that improves the dot styles. So that there is a little more space between the notehead and the first dot, and the radius is slightly smaller. So they'll be slightly smaller and farther than the flag than they are at the moment.