Start Crop Management

# Why

Proper crop management is a bit of a blocker for people who want to add pictures to crops. Issue #310

# What's Changed
* For now, if you're an admin on the site, you can edit the crop by accessing the right url `/crops/<crop_id>/edit`. This will probably change, but for now. 
* Crops have a mutation. That updates.
* Crops have a pictures field - which means they can have multiple pictures.
* An image can be added to a crop.

![screenshot 2014-12-17 13 19 43](https://cloud.githubusercontent.com/assets/485583/5467321/ed9531c2-85ef-11e4-841d-2651a1c81b39.png)

# What's next
* Build out the edit crop form to be usable.