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
It increases the learning curve for using this plugin. Would be nice to provide it by default still, to make the usage simpler, and require as less configuration as possible.

What if we check that there is `window.navigator.userAgent`, and if it is not null, then add the user agent. For server-side it obviously will be null. What do you think?