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.
```
function doFoo (callback: ng.IHttpPromiseCallback) {
  $http.get('/foo', buildFooData())
    .success(callback)
    .error(...);
}
doFoo("not a callback!"); //error
```