Fixes #2, #14, #15: Add box with explanatory paragraph and HKDemoNow FB like button

Changes made:
- Add FB SDK for the like button (inserted this at the top of `<body>` per the SDK docs)
- Implement explanatory paragraph box with number of cities calculated from the facet data. **This does not have to be manually updated.** :D

![screenshot 2014-10-01 21 43 42](https://cloud.githubusercontent.com/assets/44190/4485895/9ef9bdb8-49d5-11e4-80bb-145fe1add452.png)
![screenshot 2014-10-01 21 43 55](https://cloud.githubusercontent.com/assets/44190/4485896/9f177a60-49d5-11e4-9b57-a14a1d18ea9f.png)

Test plan:
- Safari and Firefox Aurora.

Known issues:
-   In order for the Facebook like button to show up, you have to view the page on a local webserver and not just as a file:///.