Ampersand.js - extract methods to collection

- Extract method to count total completed todos
- Listen only to relevant events in handleTodosUpdate
- Use .listenTo instead of .on
Ampersand: rename countCompleted method
Browser tests: Flatten loop
Merge pull request #1080 from tastejs/browser-test-loop

Browser tests: Flatten loop
Merge pull request #1045 from pgilad/patch-2

Ampersand.js - extract methods to collection