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 32 29 pm](https://cloud.githubusercontent.com/assets/1656824/5118104/111a6b4c-7012-11e4-8cc9-d994acacb11e.png)