Upgrading to HTTP/2 draft 14 framing

Motivation:

HTTP/2 draft 14 came out a couple of weeks ago and we need to keep up
with the spec.

Modifications:

- Removed use of segment throughout.
- Added new setting for MAX_FRAME_SIZE. Used by the frame reader/writer
rather than a constant.
- Added new setting for MAX_HEADER_LIST_SIZE. This is currently unused.
- Expanded the header size to 9 bytes. The frame length field is now 3
bytes and added logic for checking that it falls within the valid range.

Result:

Netty will support HTTP/2 draft 14 framing. There will still be some
work to do to be compliant with the HTTP adaptation layer.
Does an invalid value result in an Http2Exception of type `FRAME_SIZE_ERROR`?  I looked down the call stack here and found that `DefaultHttp2FrameWriter.maxFrameSize` throws an `IllegalArgumentException` exception but I don't see it being translated to a protocol exception.  I'm not sure if any of the other settings being out of bounds also require a Http2Exception to be thrown.