Allow dynamic settings

Hello there, 
First of all I have to say, thanks a lot for this amazing library, you've made an amazing job here.

In the application I'm working on right now I needed an option to set the appId in the .run section and not in .config. The scenario is quite simple, I need to set the appId based on the environment the application is currently running on, and in order to identify the current environment we use $location.host which is only available in .run().
using different apps for different environments is very common, for instance: https://devcenter.heroku.com/articles/facebook#1-creating-a-development-facebook-app 

When I started to use your library I noticed that I can't do that, do I modified your library to support that, basically I just added another promise that make sure we call FB.init only once appId is provided.

Let me know what you think,
Thanks again.

Asaf