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).


# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19748?src=pr&el=h1) Report
> Merging [#19748](https://codecov.io/gh/pandas-dev/pandas/pull/19748?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c8d83311611814b36aff9edab8bac902f2ec74ee?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19748/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19748?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19748      +/-   ##
==========================================
- Coverage    91.6%   91.58%   -0.03%     
==========================================
  Files         150      150              
  Lines       48862    48862              
==========================================
- Hits        44762    44750      -12     
- Misses       4100     4112      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.95% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.75% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19748?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19748/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19748?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19748?src=pr&el=footer). Last update [c8d8331...cd768cf](https://codecov.io/gh/pandas-dev/pandas/pull/19748?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks. I'm surprised we didn't already link to Brandon Rhode's tutorial.