Responsive landing page and fix for social sharing

Hey nice product by the way,

In this pull request: 
- I added media queries for the landing page
- Fix the glitch with social sharing buttons
  - [without-focus](https://f.cloud.github.com/assets/1503758/2316997/a622a586-a341-11e3-98d8-36bc19800ed9.png)
  - [with-focus](https://f.cloud.github.com/assets/1503758/2316996/a61f6ede-a341-11e3-8ab6-dc899653db16.png)


**Note:** I don't have any license to build the project on my computer, so I didn't test my fix for social sharing buttons with the app directly (just with Google Chrome inspector)