Support for non unit-testing scenarios without [ng-app] available

This fix allowed my to run my unit-tests using karma, which otherwise won't work since in a unit-test scenario no element with `[ng-app]` is avaiable. This fix makes ng-polymer-elements nobly ignore without any errors.

If I should make any further adjustment to this fix please tell me and I'd love to do it. I appreciate this project very much and it helps me to build beautiful things.

Best,

Amit.