Add "Community Favorite Crops" to home page

This PR does a couple things besides suck down time and spawn ebola jokes:

* Community favorite crops on the homepage! (based on number of associated guides, only 3 show on mobile).
* Super neat, smooth scroll, button/title thing for the featured crops
* Better CSS for the ~~super hacked~~ home nav

@rickcarlino @simonv3 Please check my actual code (things like `Routes.rb` to make sure I didn't break anything)

Hooray!
![mean kid](https://cloud.githubusercontent.com/assets/220240/4896124/f175b7a2-63f4-11e4-94b9-eb0e6904d1e3.jpg)

You guys make a good point, I'll just have to add the "hide-on-small-up" bit to each crop thumbnail rather than the rows but that's fine! Thanks for taking the time to explain this :)