Http message cleanup

Various cleanups for the HTTP message spec, as discussed on the mailing list.
The UrlInterface sounds very reasonable from a docblock angle, but I wonder how that could lead to problems in the future with cross dependencies between standards. We cant really release v2.0 of the HTTP library code to expand what the UrlInterface can do, and referencing it in another standard could be messy. 

@Crell: Could we maybe just go with string|object and mention that there object needs a toString in the meta document?