Fix version handling

Ignore frames with mismatching version numbers, except for SYN_STREAM

Per http://www.chromium.org/spdy/spdy-
protocol/spdy-protocol-draft2#TOC-Protocol-versioning

> SPDY does lazy version checking on receipt of any control frame, and
> does version enforcement only on SYN_STREAM frames.  If an endpoint
> receives a SYN_STREAM frame with an unsupported version, the endpoint
> must return a RST_STREAM frame with the status code
> UNSUPPORTED_VERSION.  For any other type of control frame, the frame
> must be ignored.
Add SPDY version auto negociation

Allow {spdy_version, negociate} to be passed to
espdy_session:start_link.

When in negociate mode, no version is selected until the client sends
its first SETTINGS frame, at which point we switch to the same version
sent in the frame.