added support for channel buffers;

Hi, 

i'm currently hacking LeapMotion and core.async;

I found your library as useful middleman between LeapJS websocket server and core.async, 
but one critical feature was missing : sliding-buffer for reading channel; 
Without that option leapjs will floot out `reading-channel` before i can even start working on it.

I added special constructor function `make-channel` and function `ws-ch` accepts now 2 optional params `reading-buffer` and `writing-buffer` to add buffers for each side.


Timo