Use default app access token in API

[ArmandoAssuncao]

Hi,

I was trying to use the default app_access_token in the Koala::Facebook::API and saw that it is not possible.

Then I looked at the code: https://github.com/arsduo/koala/blob/3c7037eea67062f05eccb16836bebec403ddbfec/lib/koala/api.rb#L95

I saw that args["access_token"] is only assigned by the @access_token. The @app_access_token will never be assigned because it does not start.

So I think it should look like this:

args["access_token"] ||= Koala.config.access_token || Koala.config.app_access_token if @access_token || Koala.config.app_access_token

Thus, if @access_token is not defined at initialize(), it will use Koala.config.access_token or Koala.config.app_access_token.

What do you think?