App access token handling

[gr8bit]

For calls requiring an app access token, use the configured app_access_token instead of taking an access token as input parameter. Deprecates the old version of these calls!

@mweibel What I did not do is wrap the app access token setter from Facebook.Config into Facebook (set_app_secret) because I couldn't think of a scenario where this would be needed... maybe you can help me with that? In which cases do you use Facebook.set_app_secret/1 or even Facebook.Config.app_secret/1?

[mweibel]

Can you fix the tests please?

[mweibel]

@gr8bit Hi, could you please update your PR and fix the tests? As to your question: not sure I understand what you mean?

[gr8bit]

Hi @mweibel! Sorry, I have been completely busy but I will catch up on this! Regarding my question: I don't know what I meant either. :D I'll get into that and hopefully understand myself again. ;)