give detail buttons 50% opacity untill highlighting cell, as discussed in #77

When highlighting a cell:

![screen shot 2013-11-19 at 17 16 58](https://f.cloud.github.com/assets/749864/1578665/7d5a2046-5181-11e3-8618-383bae253580.png)

When highlighting a button:

![screen shot 2013-11-19 at 17 17 25](https://f.cloud.github.com/assets/749864/1578667/947eb368-5181-11e3-8f2b-521a0d97f3e9.png)