Windows install instructions do not provide python3.exe, only python.exe

## Environment
* OS Version: Win 10
* Source or binary build? Source, Ionic prerelease

## Description
* Expected behavior: All tutorials use `python3` command. I should be able to follow such tutorials even on Windows.
* Actual behavior: The conda-installed python only provides `python`. When I call `python3`, it bubbles up to the Windows py.exe launcher which opens Windows Store.

## Steps to reproduce

1. Install gazebo following the official Win from source installation guide.
2. Type `python3` in console.

## Relevant info

There are a few ways to provide the `python3` command, but I don't know which one is the most suitable: https://superuser.com/a/1576801 .