Adding Composer.promisify()

This zero-arg function replaces all the async functions in Composer's
Model/Collection objects with versions that return JS promises. This
includes Model.[fetch,save,destroy] and Collection.[fetch,reset_async]

This ensures backwards compatibility while also offering the ability to
use futures in your app if desired.
Merge branch 'master' into gh-pages
adding ability to promisify composer. this preserves backwards compat while letting you use promises with your app if desired (see d37b35c)
adding promisify docs to util
fixing promisify error wrapping
Merge branch 'master' into gh-pages