README.org: Example should render, not just raw

The example in the README should show the arguments and the complete
example in the rendered version. It shouldn't just show in the raw
version.
[Fix #25] ob-sparql.el: Error on unavailable URL

Before, `org-babel-execute:sparql` would return a wrong-type-argument
error when `url-retrieve-synchronously` wasn't able to access the
provided URL. Now we check if the buffer is empty before we do anything
else and return a better error message if it is empty and the URL was
not accessible.