Callback branch

I really needed some callback abilities on my own project,
so I've added the necessary code - I hope it helps out!

Here is how to use the code:
![ss 2013-07-14 at 03 30 00](https://f.cloud.github.com/assets/1269911/793593/90fa2ed6-ec1c-11e2-8169-6e516845ca48.png)

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Here is the 'test_callbacks.html' file output - using the callbacks.
![ss 2013-07-14 at 03 28 39](https://f.cloud.github.com/assets/1269911/793587/5fe82776-ec1c-11e2-9f0d-e3fc9be20f7b.png)

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Cheers,
Agam360