New version that binds promises on then() instead of resolve()

[rochdev]

This is a follow up from last week's OTSC meeting. Basically, most if not all other implementations (zone.js, domains, cls-hooked, etc) bind promises on then(). This makes interoperability with this module very difficult for libraries that must support context propagation in multiple Node versions.

@watson Should there be some kind of v2 branch where we could merge our branch currently living in https://github.com/DataDog/async-listener/tree/datadog ?