demo(modal): add modal demo

src/modal/docs/demo.html

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+<div ng-controller="ModalDemoCtrl">
+    <button class="btn" ng-click="open()">Open me!</button>
+    <div modal="shouldBeOpen" close="close()">
+        <div class="modal-header">
+            <h4>I'm a modal!</h4>
+        </div>
+        <div class="modal-body">
+            with some content...
+        </div>
+        <div class="modal-footer">
+            <button class="btn btn-warning cancel" ng-click="close()">Cancel</button>
+        </div>
+    </div>
+</div>

src/modal/docs/demo.js

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+var ModalDemoCtrl = function ($scope) {
+
+  $scope.open = function () {
+    $scope.shouldBeOpen = true;
+  };
+
+  $scope.close = function () {
+    $scope.closeMsg = 'I was closed at: ' + new Date();
+    $scope.shouldBeOpen = false;
+  };
+};

src/modal/docs/readme.md

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+Bootstrap's modal directive.