First take at a when/callback implementation
Follow coding conventions
callback -> callbacks
'callbacks' is an object, not a single function
Simplify tests
Document callbacks.apply
Create and test callbacks.call
Define call in terms of apply
Document callbacks.call
Defined callbacks.bind
Unnecessary local functions
Don't assume that 'extraAsyncArgs' is an array
documented callbacks.bind
Handle multiple-argument callbacks
initial implementation for promisify
Support for inserting arguments at the end in promisify

The implementation got a bit hairy, I wonder if there's a simpler way...
Maintain the behavior for callbacks with multiple arguments
docs for promisify
Formatting
Removed "fillableArray" from the promisify context