Detect Visual Studio versions on the fly

[anaisbetts]

Atom build servers on Windows only have VS2010 installed. Support using either one, but prefer VS2012 if both are present.

[kevinsawicki]

Would fs.existsSync work here instead? Seems similar.

[anaisbetts]

On Windows, it is possible (though unlikely in this case) that you could determine that a file exists but you can't actually read it (because of busted permissions, your computer hates you, etc).

[kevinsawicki]

Yeah, but I thought node's fs.existsSync is implemented this same way: https://github.com/joyent/node/blob/master/lib/fs.js#L161-L169

[anaisbetts]

Legit. Replacing

[kevinsawicki]

Need parens around isWin32

[kevinsawicki]

We usually sport a little if vsArgs? instead of direct compares to null

[anaisbetts]

TIL a cool CoffeeScript hack

[zcbenz]

@paulcbetts this PR seems nearly completed, is there any blocking problem?

[anaisbetts]

@zcbenz Think I just spaced on it. I'll get it checked in today.