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)

Yeah, I've seen really weird stuff happen with fragment caching if you assign variables in a view.

Also, have you considered using `each_with_index` instead? [Check it out, yo](http://apidock.com/ruby/Enumerable/each_with_index)