adds support for multiple Paperclip attachments using Filepicker on a single Rails model

We did a bit of metaprogramming in order to support multiple Filepicker-driven paperclip attachments on the same Rails model. For example:

```
class User
  has_attached_file :photo
  has_attached_file :resume
  fileclip :photo
  fileclip :resume
end
```

The only breaking change is that the `filepicker_url` column must now be renamed to prefix the attachment name (this is more consistent with the way Paperclip names the metadata columns). Using the above example, I'd add two columns in a migration:

```
def up
  add_column :users, :photo_fileclip_url
  add_column :users, :resume_fileclip_url
end
```