DOC: Update cheat sheet and create link in docs
Update pandas cheat sheet to use `drop(columns=['colname'])`.  

Provide link to cheat sheet from the tutorials page.

Based on discussion a while back on gitter with @jorisvandenbossche , maintainers may want the link to the cheat sheet in tutorials.rst to not point to the GitHub repository as I've edited it here, but to a place within the `http://pandas.pydata.org/pandas-docs/stable/` tree, which means when you publish the docs, you have to copy the PDF version of the cheat sheet to the server hosting the docs.
