Add/send callback

I've added [`socket.send` callback](http://nodejs.org/api/dgram.html#dgram_socket_send_buf_offset_length_port_address_callback) support.

Basically, when `Client.send` is called, I'm simply checking if the last argument is a function, just before building AMP message.

I've also added a simple test and run benchmark without see any significant performance downgrade.

Punt `1.0.1` was built with callback support. I've updated to `2.0.1` (nicer), but I need this feature because I'm using punt in a script which exits after when message is sent. 

Please review.
Thanks in advance :)