Add text bubble sliders

This implements a text-bubble slider, which allows us to place the channel neutral in the middle of the slider, instead of to the right of the maximum slider value. 

1) This makes intuitive sense, as the slider goes from min to neutral to max.
2) This frees up some screen real-estate because the neutral text fields were no longer necessary.

![screen shot 2013-07-07 at 10 43 46 am](https://f.cloud.github.com/assets/1118185/767332/d437f982-e87d-11e2-9def-8698e174de8d.png)

![screen shot 2013-07-07 at 10 43 57 am](https://f.cloud.github.com/assets/1118185/767352/64d824d0-e87e-11e2-99ef-c1ab8878589b.png)