Support Dash Attributes

Minor release to add support for dash attributes in Polymer elements (e.g. <x-element out-value="{{b}}">).

Support for ng-if

General code improvements including support for the ng-if directive thanks to Jan-Willem Gmelig Meyling. Also thanks to Mark Daggett for initial code improvement suggestions.

Support Angular 1.3

Minor release to support the latest Angular:

• Tested against Angular 1.3.13
• Destroy the internal observer when the Angular element is destroyed (@heavysixer)