Support Trace and Message payloads

This supports logging of `trace` and `message` payloads. The `Rollbar`class has been refactored into a client object, where you can invoke the different types of payloads using either `Rollbar.trace` or `Rollbar.message`.

cc @astashov
You can't because "window" requires the HTML import and it won't run from the command line.

We can possibly introduce a rollbar/browser.dart import that handles something like this. But I think this should be part of a broader task.