additional visual enhancements to opacity slider

Additional enhancements to opacity slider with input from @ggeisler and @jvine

#### Before
![screen shot 2014-11-19 at 5 11 12 pm](https://cloud.githubusercontent.com/assets/1656824/5117850/19b8e4f2-700f-11e4-9084-34ed4d0cd5a9.png)

#### After
![screen shot 2014-11-19 at 5 07 17 pm](https://cloud.githubusercontent.com/assets/1656824/5117845/fe69cbd0-700e-11e4-88c8-004a271e485b.png)