HackerNews: Change the tab name from "News" to "Social"

Since reddit is using "Social", HN should probably use it too. I don't particularly like the tab name "News" because it implies that the results that I get are timely (and HN does not necessarily have timely stuff).

I also took this time to test it on mobile, and it works pretty well:

![screen shot 2014-05-16 at 4 41 32 pm](https://cloud.githubusercontent.com/assets/81969/3002209/c052687c-dd3b-11e3-9219-f74ab1e52b60.png)

![screen shot 2014-05-16 at 4 43 16 pm](https://cloud.githubusercontent.com/assets/81969/3002210/c058f6ce-dd3b-11e3-93bc-bd0b0cb282f2.png)