trait/promise: minor fix to clearing callbacks

Since promises can never change their state or value, once they have
been fulfilled/rejected, they don't need their list of callbacks
anymore.
Merge branch 'master' of github.com:behance/nbd.js
docs for trait/promise

Docs are adopting a new convention: `.method()` for methods that depend
on their context to be correct, and `method()` for methods that are
context-independent.
View/Entity: fixing a binding leak

It was possible to bind to a rendered $view, re-render, and still
trigger that binding, even though there is no way to access the old
$view.
0.2.1
Merge pull request #35 from Aintaer/master

0.2.1