#14 add return_url parameter

[Dieter Adriaenssens]

When adding a return_url parameter to the url of a form, the browser should return to that url after submitting the form.
Additional setting on the form : enable/disable this behaviour : sometimes it is not desired that the return_url parameter would override the parameter that is set in the code of the form.
When parameters are passed with this return_url parameter : check this when adding more parameters to the return url.