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
I'm not sure how useful they are. The reason I say this, is because `Rollbar.trace()`, `Rollbar.message()` and `Rollbar.traceErrorsInZone()` all returns Futures that completes with the success or failure of the API request to Rollbar. This lets the developer handle the API failure if they need to.

It also means that if the developer doesn't handle the API failure, Dart is going to report the error to the console anyway. So logging it seems duplicative. At any rate, I added the functionality back.