added an optional callback before handling an exception

This adds 2 new config options:

1. collectWindowErrors: allows you to have ranven ignore window errors, and only log the errorrs you specifically catch and pass to raven 
2. shouldReportErrorCallback: a callback that is called after stacktrace passes an error to raven.js.  If this function does not return true, the error will not be processed.  This function takes 3 parameters, stackInfo and options (from tracekit) and a callback that can be called to process the error asynchronously