Add getToken() and setToken() methods instead of class getter/setter

[joaogranado]

I think we should remove getters/setters aproach and add methods to expose/manipulate token on OAuthToken service. I think it should be the class concern to have access to the property so that the property should be updated by the container object as a side-effect of other actions. Another point I noticed with #19, is that is quite complicated to override the getters and setters using a decorator for example. I should be as easy/quick as:

function oauthConfig($provide) {
  $provide.decorator('OAuthToken', function($delegate) {
    $delegate.prototype.getToken = function() {
      // your code here...
    };

    $delegate.prototype.setToken = function() {
      // your code here...
    };

    return $delegate;
  });
}

NOTE: This won't change the service usage, and can't be considered a breaking change since these are almost private methods.