Update the frame header to make the second word always be the stream-ID.

As per HTTP/2.0 changes, # of entries is redundant information with the
length field. For non-stream specific frames, stream-ID is set to 0x0.

Note that 2 frames here had non-trivial changes:
* PING - previously the stream-ID field was a generic ID that would be
  echoed back. Now it's always 0, and the frame payload is an arbitrary
  byte sequence that gets echoed. Also introduces a new flag.
* GOAWAY - previously the stream-ID field was used for the "last good
  stream ID" field. Now it's kept as stream-ID and set to 0. This moves
  the last good stream ID field to the payload ahead of the status code.
Merge pull request #11 from willchan/StreamId

Update the frame header to make the second word always be the stream-ID.