Moved everything into a namespace called MySQLConcentrator.

I want to use this in the CiviCRM stuff I'm doing and I'm doing everything
namespaced there, so I wanted this to fit in nicely as well, so I've
namespaced it.
Assume we're running in a composer subdir.

I'm not sure what the recommended way to include composer's
vendor/autoload.php is, so for now we'll just assume that this package
is always installed with composer.
Added option to set listen port.

For the CiviCRM testing stuff I'm doing, I'm already using 3307 for
something, so I need to be able to specify a port for the concentator.
Exceptions weren't namespaced properly.
Implemented listen_port option.
Handle disconnection during auth better.

If the client disconnected during auth, the concentrator would be stuck
in a state waiting for the auth reposonse from the client and wouldn't send
the handshake init message to any new clients that connected in. Also,
it was just throwing an exception if authentication failed. Now we will just
forward on the error message to the client and treat it as if auth failed
and then the next client can try and connect.