ENH: add a carousel with examples for the front page

Adds an examples-carousel as discussed in #2333

Here's an [online build](http://jaquesgrobler.github.io/online-sklearn-build/index.html) to see it in action.



Some things I need feedback on.. 

* delay time between switching
* images to include in this (the ones I added are just for example purposes - will also use thumbnail versions thereof (smaller)
* colours of the slider-buttons:

Some options:

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![screenshot at 2013-08-02 14 32 13](https://f.cloud.github.com/assets/1378870/900889/fad9dc70-fb71-11e2-96b9-dc2e5ac1ea19.png)

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![screenshot at 2013-08-02 14 31 54](https://f.cloud.github.com/assets/1378870/900892/0cb2f0e4-fb72-11e2-8487-960322693531.png)

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![screenshot at 2013-08-02 14 31 34](https://f.cloud.github.com/assets/1378870/900895/14f4e564-fb72-11e2-95cf-15050d4e720e.png)

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Send me any links of specific examples' images you think will be good here.

Looking forward to feedback