Begin redesign, new theme and search box

This PR begins the redesign of Hex.pm by doing the following:

1. Changes Bootstrap theme to [Lumen by Bootswatch](http://bootswatch.com/lumen/)
2. Connects search box to search button
3. Changes logo to a ⬢.

I'm not sure whether we should use the unicode hexagon (⬢) or an image for the title in the navbar.

Thoughts? :+1: or :-1:?

tl;dr:
![screen shot 2014-10-01 at 4 56 51 pm](https://cloud.githubusercontent.com/assets/860934/4471344/2701a8e0-4938-11e4-9e91-4809545a6c39.png)
![screen shot 2014-10-01 at 4 58 34 pm](https://cloud.githubusercontent.com/assets/860934/4471352/6a97591a-4938-11e4-9e45-d793a3f2cbec.png)

__to...__
![screen shot 2014-10-01 at 4 56 33 pm](https://cloud.githubusercontent.com/assets/860934/4471346/2c97c096-4938-11e4-96b7-e35add580ea5.png)
![screen shot 2014-10-01 at 4 58 49 pm](https://cloud.githubusercontent.com/assets/860934/4471356/701abcce-4938-11e4-8075-2debbb3e1e33.png)