Changes from UX review for buttons on app details page (bug 1055816)

r? @ngokevin

![screen shot 2014-08-21 at 11 10 51 pm](https://cloud.githubusercontent.com/assets/23885/4006991/d6b1bfe8-29ba-11e4-8f70-41efc96c71f2.png)

![screen shot 2014-08-21 at 11 10 57 pm](https://cloud.githubusercontent.com/assets/23885/4006993/d9378f90-29ba-11e4-8a3f-c0f4e91340a4.png)