First cut of frame encoding/decoding and session management for HTTP2

Motivation:

Needed a rough performance comparison between SPDY and HTTP 2.0 framing.
Expected performance gains were seen in HTTP 2.0 due to header
compression.

Modifications:

Added a new codec-http2 module containing all of the new source and unit
tests.  Updated the top-level pom.xml to add this as a child module.

Result:

Netty will have basic support for HTTP2.
[#2358] SslHandler.safeClose(...) may not notify the ChannelPromise

Motivation:
In SslHandler.safeClose(...) we attach a ChannelFutureListener to the flushFuture and will notify the ChannelPromise which was used for close(...) in it. The problem here is that we only call ChannelHandlerContext.close(ChannelPromise) if Channel.isActive() is true and otherwise not notify it at all. We should just call ChannelHandlerContext.close(ChannelPromise) in all cases.

Modifications:
Always call ChannelHandlerContext.close(ChannelPromise) in the ChannelFutureListeiner

Result:
ChannelPromise used for close the Channel is notified in all cases
[#2361] Native.epollCreate(...) fails on systems using a kernel < 2.6.27 / glibc < 2.9

Motivation:
Native.epollCreate(...) fails on systems using a kernel < 2.6.27 / glibc < 2.9 because it uses epoll_create1(...) without checking if it is present

Modifications:
Check if epoll_create1(...) exists abd if not fall back to use epoll_create(...)

Result:
Works even on systems with kernel < 2.6.27 / glibc < 2.9
[#2362] AbstractChannel.AbstractUnsafe.write(...) is slow

Motivation:
At the moment we do a Channel.isActive() check in every AbstractChannel.AbstractUnsafe.write(...) call which gives quite some overhead as shown in the profiler when you write fast enough. We can eliminate the check and do something more smart here.

Modifications:
Remove the isActive() check and just check if the ChannelOutboundBuffer was set to null before, which means the Channel was closed. The rest will be handled in flush0() anyway.

Result:
Less overhead when doing many write calls
[#2359] EpollSocketChannel.remoteAddress0() is always null on accepted EpollSocketChannels

Motivation:
EpollSocketChannel.remoteAddress0() is always null on accepted EpollSocketChannels as we not set it excplicit.

Modifications:
Correctly retrieve the local and remote address when accept new channel and store it

Result:
EpollSocketchannel.remoteAddress0() and EpollSocketChannel.localAddress0() return correct addresses
Merge pull request #2343 from nmittler/http2

Frame encoding/decoding and session management for HTTP/2-draft10
Replace 2 spaces with 4 spaces
First round of cleanup codec-http2
[#2363] SelectedSelectionKeySet may hold strong reference to SelectionKey after Channel is closed

Motivation:
Because we not null out the array entry in the SelectionKey[] which is produced by SelectedSelectionKeySet.flip() we may end up with a few SelectionKeyreferences still hanging around here even after the Channel was closed. As these entries may be present at the end of the SelectionKey[] which is never updated for a long time as not enough SelectionKeys are ready.

Modifications:
Once we access the SelectionKey out of the SelectionKey[] we directly null it out.

Result:
Reference can be GC'ed right away once the Channel was closed.
Removing HTTP2 module's dependency on guava

Motivation:

The use of the guava library library is fairly superficial and can
easily be removed to reduce the overall dependency list of the module.
Cleaning up the HTTP2 code to use common Netty utilities, etc.

Modifications:

- Various changes to remove use of guava preconditions and collections.
- Changed default charset to use CharsetUtil.UTF_8.
- Changed StreamState to use ArrayDeque instead of LinkedList.
- Changed precondition checks to throw NPE
- Changed implementation of Http2Headers to more closely mirror the SPDY
implementation.

Result:

The behavior of the HTTP2 module will remain unchanged.
Next round of cleanup of http2 codec

Motivation:
Bring the http2 codec more inline with the rest of Netty's code-base.

Modifications:

- Add null checks in constructors
- Move private static methods into inner-classes where they are used
- Use getBytes(CharsetUtil.UTf8)

Result:
More consistent code-base
Allow the user to call slice().retain() or duplicate.retain() in his/her ByteToMessageDecoder.decode(...) method.

Motivation:
At the moment a user can not safetly call slice().retain() or duplicate.retain()in the ByteToMessageDecoder.decode(...) implementation without the risk to see coruption because we may call discardSomeReadBytes() to make room on the buffer once the handling is done.

Modifications:
Check for the refCnt() before call discardSomeReadBytes() and also check before call decode(...) to create a copy if needed.

Result:
The user can safetly call slice().retain() or duplicate.retain() in his/her ByteToMessageDecoder.decode(...) method.
Adding examples for HTTP/2 framing.

Motivation:

Provide some example code to show how to bootstrap client and server for
use with HTTP/2 framing.

Modifications:

- Fixed Http2ConnectionHandler to allow headers after stream creation.
Needed for response headers.

- Added toString() to all frame classes to help with debugging/logging

- Added example classes for HTTP/2

Result:

HTTP/2 connections now properly support response headers. Examples for
HTTP/2 provided with the distribution of examples module.

After your change, what will change.