OAuth2.0 Implementation

[HelNershingThapa]

This PR adds changes to implement the OAuth2.0 authorization framework.

Summary of changes made:

• Removed functions that helped to generate signatures. timenonce(), getAuth(), etc. that were used to generate signatures have been removed since they are no longer required. OAuth 2 relies on HTTPS for security, we don't have to "sign" requests anymore.
• Added authorization data to request headers.

[bhousel]

This is pretty great @HelNershingThapa -
Because OAuth2 is much simpler - relying on bearer tokens and SSL for security - it means that almost nothing remains in this library after the proposed change.

So I think what I'm going to do is:

• just keep this ohauth library as a "most-of-the-way OAuth 1.0 client implementation in Javascript".
  It will become less relevant over time and that's ok.
• but move the few remaining things (xhr, header generation) back over to the https://github.com/osmlab/osm-auth library, which will be focused on OAuth2 going forward.

[HelNershingThapa]

That sounds great. I'll be looking forward to the new release of the osm-auth library. Thanks for the review @bhousel

[Ichchhie]

Thank you @bhousel and @HelNershingThapa for the collaboration. This looks exciting. Really looking forward to the new release of the osm-auth too :)