Root amq-client in the amqp gem repo

This is our first step to getting rid of amq-client.
It was a good idea 2 years ago but now it's clear that
there's next to no value in it for clients that do not
try to mimic amqp gem's API, such as Bunny 0.9.

Bunny and amqp gem already reuse amq-protocol, but their
networking and public API parts are too different to try
to extract a common library.

So long amq-client. You've served amqp gem well for good
2 years.
Remove an example group that does not make much sense
Don't use a method that has been renamed a while ago
Begin inlining amq-client code

AMQP::Queue no longer extends AMQ::Client::Async::Queue and includes
all its code instead.

This and remaining steps will make amqp gem code structure much
more straightforward to navigate and will remove some dead-by-now code.
Ditto for AMQP::Exchange
Missing from the previous commit
All extensions stuff is already in the core
README updates
README updates
Inline AMQ::Client::Channel
Remove Cool.io connection adapter
branding, yo
Inline AMQ::Client::Consumer
Remove now dead code
Remove now dead code
Move Openable and Entity to AMQP::
Move HandlersRegistry to AMQP::
This file is now under amqp/
async/adapter
Two more entities have been moved to amqp/