Added 'bind' method to channel to bind to a class as a handler for messages
Simpler implementation of map, filter, reduce and group
Updated channel tests
BUGFIX: Retry was broken by change f3ead587e0c0a52e8791babe0b5c16ad261be5bf

Though regular phi jumps should be ignored, retry steps should
not be ignored during error unwinding.
v0.4.0