Fix docs security

Initial attempt at improving documentation (for issue #800).

The patch already uses SSL for PyPI urls (the official certificate will be deployed in the next few days).

For global install, since on Linux there is a very secure path (using the system package manager), I highlighted this path, and refactored the rest of the doc to be a general "on other operating systems".

For Mac, homebrew docs say that to get pip one should install their Python version, or alternatively use easy_install. I will submit a patch there as well, suggesting to use get-pip.