Simplify settings and hide SPDY Variant construction.
Merge pull request #479 from adriancole/hide-settings

Simplify settings and hide SPDY Variant construction
Fixed bad length bugs in http/2 reader, ignore unknown frames, and cleaned up frame handling.
Merge pull request #478 from adriancole/harden-reads

cleanup frame handling in http/2
Backfill spdy and http/2 related tests wrt zero-length messages.
Merge pull request #480 from adriancole/backfill-tests

Backfill spdy and http/2 related tests wrt zero-length messages.
Create concurrency doc.
Improve some Markdown
Style fixes
Honor connection-level flow control for writes.

We don't write until the peer has given us a budget.
Merge pull request #482 from square/jwilson_0126_flow_control_writes

Honor connection-level flow control for writes.
Honor change in connection-level initial write window from peer settings.
Merge pull request #483 from adriancole/connection-settings-changes-write-window

Honor change in initial write window from peer settings.