Add pandas cookbook to tutorials (for #5837)

@jreback asked me to add something like this in https://github.com/jvns/pandas-cookbook/issues/1

I haven't been able to test that this displays correctly as I can't figure out how to build the Sphinx docs.

If someone can tell me how to build the docs, I can test it.

```
bork@kiwi ~/c/p/pandas> pwd
/home/bork/clones/pandas/pandas
bork@kiwi ~/c/p/pandas> python ../doc/make.py html
Error: Cannot find source directory.
Building HTML failed
```

```
bork@kiwi ~/c/p/doc> pwd
/home/bork/clones/pandas/doc
bork@kiwi ~/c/p/doc> python make.py html
Running Sphinx v1.1.3
cannot import name hashtable
Exception occurred while building, starting debugger:
Traceback (most recent call last):
  File "/opt/anaconda/lib/python2.7/site-packages/sphinx/cmdline.py", line 188, in main
    warningiserror, tags)
  File "/opt/anaconda/lib/python2.7/site-packages/sphinx/application.py", line 102, in __init__
    confoverrides or {}, self.tags)
  File "/opt/anaconda/lib/python2.7/site-packages/sphinx/config.py", line 216, in __init__
    exec code in config
  File "/home/bork/clones/pandas/doc/source/conf.py", line 74, in <module>
    import pandas
  File "/home/bork/clones/pandas/pandas/__init__.py", line 6, in <module>
    from . import hashtable, tslib, lib
ImportError: cannot import name hashtable
> /home/bork/clones/pandas/pandas/__init__.py(6)<module>()
-> from . import hashtable, tslib, lib
```