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.
Comment should be on on line 461 (but not close enough to a change to allow a comment). Should the constructor of the Http2Settings object be passed `6` instead of `5`?  Use a constant.  Also investigate the use of the default constructor and if it is appropriate to use this size for the default constructor.