DOC: Add recipe for shifting groups of values based on the index
So edited.
 @unutbu thanks!
 It think in 0.14 enabled `.shift(1)` to work directly, rathern than `.apply(lambda grp: grp.shift(1))`

i'll fixup
 Ah. That works as far back as 0.8.0 as well.
 Just for fun I wanted to use `git bisect` to find the first commit where `groupby(...).shift(1)` works. I can do it manually, but this requires that I reinstall pandas to test each commit:

```
/bin/rm -rf ~/src/pandas/{build,dist} && cdsitepackages && /bin/rm -rf pandas*
cd ~/src/pandas && python setup.py build_ext --inplace && python setup.py install 
```

I suppose I could write a unittest script with a setUp method which performs the installation, and then run `git bisect run ...`. But is there an easier, automated way?
 ahh...that's the automatic function call generation, kind of a syntactic sugar

I don't any easier way to run git bisect....only used it a couple of times.

I think bisect will do the install for you (e.g. it checkouts a particular commit), runs setup, then runs the code....
