added pd as namespace
more consistent with code in the rest of the document

@dacoex yeh, you will need to explain why you want to add a warning like that (in another PR). 
 @jreback seems that you are not convinced of the usefulness.
 I am talking about the keys accessor warning. just take out for now and you can put in another PR. (e.g. just the `pd.` changes here.

No I am not convinced its something people should be doing at all. The attribute access is just a convenience feature, string accessing by paths is the preferred method.
 OK, now this PR is only about the namespace changes.
 can you rebase this
 pls don't open new pull-requests for the same fix, just force push to the same. see the [docs](http://pandas.pydata.org/pandas-docs/stable/contributing.html)

simply

`git push yourbranch yourremote -f`
