Supply complete Host header in tests.

Per RFC 2616 §14.23, the Host header can include the port number.
Respond with 400 Bad Request for bad Content-Type.
Respond with 400 Bad Request for missing/unparsable parameters.
Split YoudoData off from Youdo.
Add newtype YoudoID
Add newtype UserID
Add transaction ID to Youdo.
Add newtype WebYoudo for JSON conversion.
Respond with 405 Method Not Allowed to wrong method on /0/youdos
Add `resource` for handling 405 responses.
Improve variable name in maybeError.
Use `resource` for /0/youdos/:id
Add GET /0/youdos/:id/versions
Add updating of Youdos.
Include colon in base URI scheme, as uri lib expects.