DOC: Updated tutorials.rst, added 3 recent video tutorials
I have made the following changes to  `tutorials.rst` (found [here](http://pandas-docs.github.io/pandas-docs-travis/tutorials.html)):
- Updated the version of the [pandas-cookbook](https://github.com/jvns/pandas-cookbook) from `v0.1` to `v0.2`.
- Added an introductory paragraph to the tutorial series by @TomAugspurger.
- Added 3 recent, high-quality video tutorials (Youtube).

