Fixed code style issues

Added .jshintrc from ToDoMVC https://github.com/tastejs/todomvc/blob/master/.jshintrc
The code which was taken from underscore is jshint ignored rather than fixing jshint violations.
Moved fetchIssueCount function to the Learn prototype
The new 'issues' section in the learn bar is hidden by default

The issues section is only shown when the API reports >0 issues for an implementation
Added null check to make migration easier

This check means that the script will not fail if learn.json is not updated at the same time as todomvc-common