Add filepicker_image_url to work with policies

Closes  #93 

@bcackerman do you can test the policies with this branch? As you [can see now it is appended the signature and policy to query](https://github.com/Ink/filepicker-rails/blob/13775bae12decb190eb706d75cca0a65413d3c8a/spec/helpers/aplication_helper_spec.rb#L175-L186) as documented [here](https://developers.inkfilepicker.com/docs/security/#usePolicy).

Please test this with a convertions and without a convertion.