Integrate modal login screen

### Story ###
- https://www.pivotaltracker.com/story/show/83857244

### Notes ###
- added new modal login screen to devise layouts as partial
- ```home#index``` is no longer authenticated, Ember views are rendered without logging in
- failure to sign in redirects back to root path

### Screenshot ###
![screen shot 2014-12-10 at 12 13 39 pm](https://cloud.githubusercontent.com/assets/456952/5380447/2c68e50e-8066-11e4-9686-f8833fc93c0a.png)