Update and clarify installation documentation

[efiring]

As noted in #2773, the installation section of the FAQ is out of date, and our installation documentation in general does not give sufficiently clear and correct advice, particularly for OSX and Windows users.
My inclination is to strongly recommend that non-expert OSX and Windows users take advantage of either Enthought or Continuum.io.
My impression is that if they really don't want to do that, then the recommendation is to first install python from python.org, and then for everything else use pip and hope for the best.

[tacaswell]

Also see #3001 for osx install documentation issues.

[tacaswell]

Is this closed by #3122?

[efiring]

No, it isn't. Additional updating is needed to reflect whatever OSX binaries will be provided with this release, and even more than that might be in order. Whoever is going to be in charge of providing Windows and OSX binaries needs to handle this part of the updating.

[tacaswell]

This should also deal with github deprecating their download page and what ever they replaced it with.

[tacaswell]

@efiring Please re-open this if you don't think #3197 is sufficient..