Fix downloads link for starter-kit

The link to downloads is deprecated.
* https://github.com/blog/1302-goodbye-uploads

Now, we can download tagged version from '/tags'.
Merge pull request #2689 from tricknotes/fix-downloads-link-for-starterkit

Fix downloads link for starter-kit
Find helper now throws when element not found and added selector context
Merge pull request #2690 from teddyzeenny/find-helper

Find helper now throws when element not found and added selector context
Merge pull request #2692 from stefanpenner/master

Ember.computed learns oneWay computed
update doc example to current router