update style and functionality of layer opacity widget

Makes opacity control more usable by setting background area and updates styling. @tcramer suggested style updates.

Closes #126 

### Before
![screen shot 2014-11-17 at 1 43 29 pm](https://cloud.githubusercontent.com/assets/1656824/5078232/caa26e98-6e5f-11e4-8320-e54359ee29f2.png)

### After
![screen shot 2014-11-17 at 1 43 37 pm](https://cloud.githubusercontent.com/assets/1656824/5078236/cf896290-6e5f-11e4-877a-2da6db674f16.png)