tasks/build_task.js: Add copy:azure_dashboard_to_default

Setup future:

  $ grunt build

runs to automatically install azure.json as the default dashboard.
This catches the build-tooling up with 35ede07
(src/app/dashboards/azure.json: Rename our default dashboard,
2014-01-22).

I'd rather this commit immediately followed 35ede07, but the build
tooling has changed a good deal since we initially branched off from
a59f9e6 (Added filesaver module to dashboard service. Closes #503,
2013-09-16), and adding this commit here saves us some conflict
resolution trouble.