AngularJS: fixing http promise typings

Looks like defining only a single overload of the then function on the
IHttpPromise interface is making the other overload of it defined on
IPromise invisible to the compiler. As such, we need to expose both
versions. Otherwise, the standard promise unwrapping behaviour does not
type check correctly.
Merge pull request #985 from anchann/angulartyping

AngularJS: fixing http promise typings