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.
There is a TODO further down that reads:

> // TODO: consider adding a flush() method to this interface. The
            // frameWriter flushes on each write which isn't optimal
            // for the case of the outbound flow controller, which sends a batch
            // of frames when the flow control window changes. We should let
            // the flow controller manually flush after all writes are.
            // complete.

We should likely address the TODO's now that we are approaching a more final state.  The description here seems beneficial, what are you current thoughts?  Is this something we want to create a follow up issue for?