This PR restructures the Headroom Directive:

• Uses a .factory() to return the global dependency and register within Angular DI system
• Allows HeadroomService to be dependency injected
• Annotated dependencies for the Directive to prevent non-build system minification errors (such as ngAnnotate)
• Use Angular's internal $document wrapper over window.document Object
• Fixes a bug with if (options.scroller) that causes [object Window] error
• Replaced angular.forEach with native Object loop
• Reduced $destroy callback function to just headroom.destroy argument

One thing we need to discuss is integration of the isolate scope properties (scope: {} etc):

• Right now these are two-way binding instructions to Angular using =, however this will not update the right properties currently inside the headroom instance. One way to achieve this is setting up $scope.$watch on each property and registering the correct callback and property setter to achieve desired results.
• Some unit tests can be added but for an integration this size and with such simplicity I'm going to say don't bother

@toddmotto