add ALTSVC frame type.
rough in opportunistic
add blurb in intro about ALTSVC
Mention alt-svc section 2.4.
alternate -> alternative
make altsvc ref normative
use AltSvc reference
remove HTTP:// over TLS for now
add individual origins to ALTSVC
Only allow a single origin in ALTSVC
Tweak frame layout; explain what to do with Reserved.
Rearrange list of ALTSVC fields to match illustration ordering.
Merge pull request #439 from http2/altsvc

Integrate Alternate Services