Extract default settings to variants.  Read windowSize (spdy/3) and headerTableSize (http/2) from peer.
Merge pull request #456 from adriancole/read-settings

Settings: Wiring, Variant-specific defaults, and HTTP/2 acks
In HPACK, specifying the index of an referenced entry dereferences it.
Merge pull request #457 from adriancole/hpack-toggle

In HPACK, specifying the index of an referenced entry dereferences it.