Added handling of ACK for config commands.

We now have two classes of commands: blocking and non-blocking.
`CONFIG` is blocking, all others are non-blocking.  When a blocking
command is sent, no other blocking commands will be sent until an ACK
is received (and cleared) from the AR.Drone.
Fixed missing var.
Added unit tests for CONFIG ACKs.
Changed signature of config.

config can now be called as config(key, value, callback) or config(options).

Added more info on config to README.
Fixed config signature to bring callback out of options.
Increased default timeout to 500 ms.

Also removed vestigal retries references.
Increased delay in test to trigger timeout.
Merge pull request #76 from wiseman/config_acks

Added handling of ACK for config commands.
0.3.0. Backwards-compatible new features for Client.config.