Add sinatra backend

[JAndritsch]

Hey there,

I've finally gotten around to forking your uploader and adding my sample Sinatra backend. I haven't done anything with upload persistence, however, so this example is purely for generating signatures.

Let me know if you have any questions and please feel free to cleanup the sample as you see fit.

Cheers,

Joel

[gabipurcaru]

Thank you very much for the pull request, I've changed your code a bit to automatically render index.html (as a ruty template, compatible to jinja/django/etc) and static files, so that you can use ruby example_backend.rb and just go to localhost:3000. Also I've moved the backend to /ruby_backend/example_backend.rb.

Again, thanks for the help!