update handle-response to return result info, and unpack in invoke
slacker. exception won't be thrown from SlackerClient. to fix
https://github.com/sunng87/slacker-cluster/issues/5
update async call API:

1. callback now accept two argument: [error result] (breaking change)
2. async functions returns custom type of Promise, will throw
exception on deref if error.
also update inspect to return call-result.