AngularJS: give an interface to $http callback functions

- add IHttpPromiseCallback for both `error` and `success` promise calls

use case is if we're passing around the callback, we don't want to be
repeating that entire function declaration every time, and we cannot use
`Function` as that doesn't satisfy the compiler. (see test case)
Merge pull request #797 from Hudon/master

AngularJS: give an interface to $http callback functions