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'll add "language" back and possibly "timestamp". The others assume too much on the environment. It's better to keep it generic so that this can be used in both browser and server apps.