Update to h2-11, refactor Frame and subclasses to remove duplicated code
Convert Frame.has_stream to a string-based enum and rename to stream_association
Use correct MRO for Frame mixins, fixup constructors
Specify bitmask in hex notation, rather than bit offset
Avoid a data buffer copy in PushPromiseFrame
Split out AltsvcFrame.serialize_origin() and parse_origin()
Rename AltsvcFrame -> AltSvcFrame
Merge pull request #47 from alekstorm/h2-11

Update to h2-11, refactor Frame and subclasses to remove duplicated code