Add shouldSendEvent callback

[HazAT]

This adds a shouldSendEvent callback the be able to prevent sending of specific events.
e.g.:

sentryClient.addShouldSendEvent(new ShouldSendEventCallback() {
            @Override
            public boolean shouldSend(Event event) {
                // We don't want to send events that are from ExceptionsManagerModule.
                // Because we sent it already from raven.
                if (event.getSentryInterfaces().containsKey(ExceptionInterface.EXCEPTION_INTERFACE)) {
                    ExceptionInterface exceptionInterface = ((ExceptionInterface)event.getSentryInterfaces().get(ExceptionInterface.EXCEPTION_INTERFACE));
                    if (exceptionInterface.getExceptions().getFirst().getExceptionClassName().contains("JavascriptException")) {
                        return false;
                    }
                }
                return true;
            }
        });

[bretthoerner]

I made it a private set of callbacks instead of a single static one, looks good to me now!