Client settings during Upgrade-based negotiation

#51:Client advertising settings during Upgrade dance
https://github.com/http2/http2-spec/issues/51
address some comments on this pull

IANA section. Short blurb on why this header exists, and clarifying that
an empty header is fine.
Indicated the SETTINGS that must be communicated
Marking this as a hop-by-hop header.
Merge pull request #146 from GabrielMontenegro/master

client settings during Upgrade-based negotiation