Modified UI for event show page

PT story https://www.pivotaltracker.com/story/show/75430084

Make Join link a button
Add description to Start Hangout button
Adjust spacing for event description
Adjust spacing for hangout details

Demo at my [my instance](http://yaro.herokuapp.com)

![image](https://cloud.githubusercontent.com/assets/5670940/3672223/eadad232-125b-11e4-8f47-07def554aba3.png)