Payment channel implementation sketch

Simplifications:

 - unidirectional paychans
 - no top ups or partial withdrawals (only opening and closing)


 TODO
 - chnge module name to "channel"?
 - Find a better name for Queue - clarify distinction between int slice and abstract queue concept
 - refactor queue into one object
 - Do all the small functions need to be methods on the keeper or can they just be floating around?
 - Tidy up - standardise var names, method descriptions, heading comments
 - any problem in signing your own address?
 - Gas
 - Codespace
 - find nicer name for payouts
 - tags - return channel id