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
Ah, right, there is no way to check variable existence in Dart... Okay, let's postpone that. It kinda sucks though that users will have to specify that `{client: {javascript: {browser` thing every single time, especially because majority of Dart apps are dart2js apps.