feat: export static GoogleAuth

[JustinBeckwith]

Addresses #4

[JustinBeckwith]

My initial approach here was to try using es6 default exports:

export default new GoogleAuth();

However - given the lack of compat between CommonJS and es6 modules, consumers would have to use this syntax:

const auth = require('google-auth-library').default;

That doesn't feel terribly discoverable, and isn't much better from what we have today. Webpack works around this, but apparently TypeScript isn't really planning on muddying the waters:
microsoft/TypeScript#2719

Instead, I ended up taking the named export approach. I export both the GoogleAuth class, AND an auth object with a single instance. It means consumers using Node 6 and above can do this:

const {auth} = require('google-auth-library');
auth.getApplicationDefault()...

The module.exports and exports = syntax aren't terribly es6 compatible, and would have caused a whole other set of issues.

This seems like a fair compromise, but I'm looking for opinions :)

[JustinBeckwith]

@ofrobots and @MylesBorins, would love to hear your thoughts too :)

[ofrobots]

Instead, I ended up taking the named export approach.

This the right way to go at this point; given uncertainty about how ES6 interop is going to be settled. Regardless, default exports are banned by the style guide.

LGTM w/ nit.