Improved API of http_request and minor improvement of http_response.

Added methods to access most request parameters (from 'meta'). Methods 'header', 'cookie' and 'arg' (old 'request', new name is more meaningful) return boost::optional now, that reflects optional nature of these values. In http_response added method set_content_type, that adds content-type header or updates if it already exists.
Remove some useless methods of 'http_handler'. 'error' method sends error to engine, not http error. Also user can throw exception from handler if something fatal is occured (and worker send error to engine himself), so i think that 'error' method is confusing in http application. Also it seems that users will fill http_response themselves (this includes setting of http code), so 'ok', 'bad_request', etc methods looks useless.
Merge branch 'http2' into v0.10