Adds support for AMD loading

Allows the script to be loaded via AMD, e.g. require.js.

**Requirements for require.js:**

Add pathis for `FBAngular` and `angular` named modules to your config:

    var require = {
      /* ... */
      paths: {
        'angular': 'path/to/angular',
        'FBAngular': 'path/to/angular-fullscreen'
      }
    };

Add `FBAngular` to your module's dependencies:

    define(['angular', 'FBAngular'], function(angular, fbAngularModule) {
      // Note: fbAngularModule is the angular module, but most likely isn't needed

      // Add 'FBAngular' as a dependency to your Angular app:
      var app = angular.module('myApp', ['FBAngular']);
    });

Additional changes include:

- Use inline bracket notation to allow the file to be minified, see: http://docs.angularjs.org/tutorial/step_05#controller_a-note-on-minification
- Inject `$document` instead of using global `document` to improves testability
Allow full-screening an element by watching a $scope property

This allows any element to go into full-screen mode without having to use IDs. Simply toggle a property on the `$scope` and pass in the property's name to the directive.
Merge pull request #2 from djvirgen/watch_scope

Allow full-screening an element by watching a $scope property