Upgrade Image Handling to Use S3

##### Fixes #130: Upgrade Image Handling to Use S3
- [x] Upgrade Image Handling to use files instead of URLs (#88) 
- [x] Create an S3 account (@DBNess)
- [x] Update Paperclip config ([Heroku help doc](https://devcenter.heroku.com/articles/paperclip-s3))
- [x] Migrate currently used images to S3