Added modal close button in header by default

Issue
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There is no close button on modal by default, cannot close or go back, asking user to add button with `onclick="$.ui.hideModal()"` is too manual

Test Case
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Add a new modal panel: `<div title="Modal" id="modal" class="panel" data-modal="true"></div>`, this will just open a white full screen div and cannot go back

Fix
-

`appframework.ui.js`:
Added a header with close button by default and no footer
User can still override this by adding their own `<header>` and `<footer>` in the modal
adding `data-header="none"` will remove the header in modal
added `.panel` class to modal content to get consistent style as other panels

`index.html`:
added demo for default and custom modal window
added a button in custom header

![screen shot 2014-01-23 at 3 45 52 pm](https://f.cloud.github.com/assets/1414842/1990618/fdfe39ec-8487-11e3-8f1a-f497b9dc4617.png)