Included a RavenCallback global that allows loaded callbacks

We are loading RavenJS asynchronously, which means we don't know when it will be loaded.

That makes it very hard to call `config` and `install` reliably.

This patch creates the following use case:

    window.RavenCallback = function(raven) {
        raven.config(myDSNUrl).install();
    };

    // code to load raven asynchronously

This will be a great addition to an already great library.