Add 'angular.isPromise' reference check

Adds a check for whether an input is a promise/deferred object. 
I've found this useful when I want my functions to accept either X or a promise of X.

Example usage:
```javascript
var promise = $q.when({ some: 'object' });
angular.isPromise(promise); // => true

var notPromise = { some: 'object' };
angular.isPromise(notPromise); // => false
```