Add os.homeDir()

[domenic]

In the spirit of #3407, this would be nice. From npm:

var home = ( process.platform === "win32"
           ? process.env.USERPROFILE
           : process.env.HOME )

Example of where this would have helped people write Windows compatible modules: browserify/browserify@bf0354a

Can do a PR later tonight if you guys are agreeable.

[isaacs]

I'm feeling kind of iffy on this. Even having os.tmpDir in there is kind of bugging me.

I don't want to leave you hanging though, so I wrote this for you to use instead: https://github.com/isaacs/osenv