Add button to hide >>> and ... from code examples

All code blocks in the documentation will now have a button added to it at the top right, that allows the user to hide/show the `>>>` and the `...` characters, such that the code becomes executable.

Note that I am aware of IPython's magic that allows you to copy/paste code with `>>>` in it and successfully ignore it, but I think this comes in handy for people that do not use IPython (even though they should) or when you are copying code into a text editor. Plus other projects have this in their documentation.

~~Mandatory~~ GIF demonstrating the feature:

![copybutton](https://cloud.githubusercontent.com/assets/375307/2809287/171cfc12-cd65-11e3-829d-feb20a3e7717.gif)


-----

Credit goes to scikit-learn and to python.org, will add any other credit references that people think I should.