A few updates that I found made working with this module a little easier

Hi David,

Thank you for creating a `passport` strategy built on the official `googleapis` module = more future-proof!

This PR contains the following updates:

* Add `provider` attribute to user profile - helps differentiate users when there are multiple strategies - this follows the pattern at http://passportjs.org/guide/profile/
* Ensure wrapped errors contain the upstream message - no more "[object Object]".
* Correct README usage examples - the upper-case ID of the incorrect `clientID` may have originated from the original `passport` OAuth module.
* Ensure `npm` scripts work cross-platform - this keeps Windows users happy (well, a bit happier) and also ensures the local version of `jshint` _et al_ is used in preference to anything installed globally.
* Version bumps - shiny new version of `mocha`.

Cheers,
Lovell