Added doc for spdylay_error values

This callback function is invoked after the control frame
is not sent because of the error. The error is indicated by
the error argument, which is one of the values defined in spdylay_error.

The callback functions receives SPDY/3 style name/value pairs.

translate them into SPDY/2 as needed.

of the request submitted by spdylay_submit_request().

spdycat should fail for failed requests

Ensure that date and content length strings are still valid when they are put into the name values list.

SPDY versions is selected.