Handling of schema uri

[koenedaele]

Currently the constructor accepts the following connected parameters: base_url, vocab_id and url.

The url is a composition of base_url and vocab_id, but currently the vocab_id is mandatory. I should be able to just set the url and not have to worry about the vocab_id.

I'd also rather rename a few of these parameters to make them more clear. Please rename url to scheme_uri, base_url to base_scheme_uri and vocab_id to scheme_id.

Please include the schemes/ in the base_scheme_uri.