Resources and token expiry

Changes:

• The module now exports a createResource method that returns a resource object with convenience methods

Fixes:

• Important: Access tokens now expire and renew automatically
• Improved error handling
• Improved testing

v3.0.0

Changes:

• Breaking: The module now exports a constructor
• execute() now also passes the status code and headers to the callback

Fixes:

• Unique access token promise fixes a race condition issue when trying to authorize multiple parallel requests
• Improved error handling
• Improved testing