Remove distinction between `url` and `save_url`

[sweh]

In order to get the Form-API more REST friendly, the save_url parameter should be replaced with a generic url parameter for save and load of data.

For backwards compatibility, the first parameter to the Form.load() function should stay as is (data or url), except that providing a url as data and a url in options should result in an error.

If data is provided directly, url must be provided in the options.