Issue 4

[dariobauer]

Fixes and improvements to the validation of authorization codes and access tokens.

• Moved the access token check from the _create_headers method to the _get_token method as it makes more sense to check it there;
• Fixed the access token check to look for a json web token rather than the length of the string, resolves issue Access token length check is too strict #4;
• Update the auth code extraction to use regex rather than make assumptions on the position within the returned url.