Support Arbitrary Errors

The previous version of the message parsing and encoding made it hard to support other kinds of messages. This pull request changes how erollbar handles message and introduces the notion of handlers.

Handlers are funcitons with arity one that convert incoming messages from the `error_logger` to erollbars internal message structure using the `erollbar_message` API. The library is then able to ship these internal messages out to Rollbar.com when needed.

Handlers deprecate the filters, as handlers can choose to ignore messages and can as such double as filters.
What are the undefined fields there? What should I expect?