rss, json and more

Newb alert: This is my first pull request!

I've piggybacked on [another fork](https://github.com/joelvh/rails_admin_import) which includes the ability to configure a lookup field. This means that you can update or create a new record, depending on some field. 

Things I added
1. You can import from a url, instead of uploading a file.
2. You can import json data. 
3. You can import from a rss feed. You'll need to configure this mapping in rails_admin.rb.
3. I rearranged the view. This includes helper links in the message. The filetype is set explicitly, not inferred from the content type. 

![import](https://f.cloud.github.com/assets/474792/288581/06e43c50-927b-11e2-8b2b-e2ba88b580cb.png)