First pass at image upload

Greetings, 

Took a stab at #8. 

For this PR I:
  * Created a new model (Image) belonging to Crop
  * Added a has_many to Crop
  * Added a new nested POST route under crop for images for form submit
  * Added a new controller Images to handle this POST
  * Added an 'Add Image' link to the bottom of the crop table (I18n fo en included)
  * Created a modal to display a file_upload field
  * Display images in a slick carousel - Foundation states theirs is deprecated

Looking forward to your feedback!
 
![openfarm 2014-10-08 19-03-38](https://cloud.githubusercontent.com/assets/334550/4568812/62204352-4f3f-11e4-856d-2e9c6da9009c.png)