Prefer resource_url instead of model_url

[miohtama]

Pyramid has changed its vocabulary to use resource_url instead of model_url:

   For backwards compatibility purposes, this function can also be imported as model_url, although doing so will emit a deprecation warning.

This change makes resource_url preferred Jinja2 filter.

Furthermore the changes include cleaning up documentation and moving filter examples to API documentation, so that IDEs can pick them up from there.