Change `setup.py install` recommendation to `pip install .`

I think this [comment](https://github.com/numpy/numpy/issues/6551#issuecomment-154561362) makes a lot of sense.

We should update our docs to recommend `pip install .` where appropriate.  We can't (and shouldn't) explicitly disallow `setup.py install`, but most users will have a better time of it using `pip` to install from source in this day and age.
