Services for HTTP methods PUT and DELETE

HTTP methods PUT and DELETE are necessary to develop RESTful APIs. This patch replicates `post_{service,coservice,coservice'}` for these methods (`put_service, delete_coservice`...).

It's a rather dumb copy & paste & replace of POST-related functions, but it works. An Eliom guru could probably review it, check there's no mistake, and factorize some code between POST, PUT and DELETE functions if possible.