Make Meteor.wrapAsync official, and simplify binding.

Summary:
To bind the wrapped function to an object, just pass that object as the
optional second argument to Meteor.wrapAsync:

  var response = Meteor.wrapAsync(HTTP.get, HTTP)("http://example.com");

Test Plan: meteor test-packages meteor

Reviewers: emily, avital, sashko

Reviewed By: emily

CC: nim

Differential Revision: https://phabricator.meteor.com/D807