Support multiple instances

[lokshunhung]

Goal

Resolves #876

Implementation

• Instead of having a module-scoped gcmAPIKey and vapidDetails in src/web-push-lib.js, we change those to instance properties of a WebPushLib class instance.
• When re-exporting the setGCMAPIKey, setVapidDetails, generateRequestDetails for the global instance of WebPushLib, we need to .bind() the methods to retain the this context.
• Also export the WebPushLib constructor so users can create multiple instances instead of just using the global instance.

[marco-c]

@lokshunhung could you add a test for this?