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.
Interesting question.  Is this something that we should make configurable?  I'm not sure if we can generalize the answer if using the same padding across multiple packets is better or not (it may depend upon the actual payload).  Would it be worth it to introduce a new interface (and default class) that determines how the padding is distributed when the frame is split?  I can think of a few different cases:
1. Do what your doing here, (i.e just apply the padding on 1 frame)
2. Same padding on on splits
3. Some partition of padding goes into each frame
4. Splitting a frame creates an interesting new case to generate different padding for each frame?

This may be a pain to pipe in the new interface but I'm not sure if we can generalize the correct choice.  We would only need to provide 1 implementation which could be just what you already have.