Add "Message" tab to examples that use a message

These changes allow examples to contain a `meta.yaml` file, in which `messageId` can be defined, and when the example is rendered, a "Message" tab will automatically display the ICU Message string for the current locale.

![screenshot 2014-10-03 15 15 09](https://cloud.githubusercontent.com/assets/29096/4511168/9bd13b4a-4b31-11e4-8208-27047dc8a42d.png)