Added get method to $q

This adds a nice utility method present in [Q](https://github.com/kriskowal/q/wiki/API-Reference#wiki-promisegetpropertyname) which will allow you to fetch a particular key's value from a resolved/rejected value of a promise.

This is useful for doing stuff like `$http.get(url).get('data')` instead of having to do `$http.get(url).then(function(res) { return res.data; })`.  In addition, we can do

<pre>
promise = foo.bar();
newPromise = promise.get('reallyreallyreallyreallyreallyreallyLongKey')
                    .then(moo);
anotherPromise = promise.get('anotherreallyreallyreallyreallyreallyLongKey')
                        .then(foobar)
                        .then(bar)
                        .then(baz);
return $q.all[newPromise, anotherPromise];
</pre>