Node.js installation instructions/information is different for OSX, Linux and Windows

[rwaldron]

On http://tessel.github.io/t2-start/index.html

Of course there are specific, platform relevant instructions for each, but ideally these would have mostly uniform language. Also, we need to specifically call out the Node.js version.

[Frijol]

Very good point. We should absolutely fix this.

[dmoneyballer]

Just to Clarify, you want to show like on windows in command prompt run: npm install -g t2-cli and in linux's terminal you should run sudo npm install -g t2-cli after installing Node.js?