Use rounded rectangles to draw notes

Improve the drawing method of the notes using a rounded rectangle, it is simpler and (at least for me) looks better.

Screenshot containing notes with different lengths, volumes and panning:

![2014-12-26-151211_416x349_scrot](https://cloud.githubusercontent.com/assets/347552/5558524/113092d8-8d12-11e4-984e-8a77af3582e2.png)

As @diizy pointed out on #64 this will be refactored into a collection of views instead of drawing the notes, but at least this is an improvement for the time being until it gets implemented. (I wanted to make it myself but I was having too much trouble).