Handle messages differently, using handlers. This
makes is a lot easier to support different error
messages, and something that the library
maintainer doesn't need to do.

Introduces structured internal messages that handlers
create. These structures are the encoded into
JSON which Rollbar.com understands.
Add a ranch handler.
Be explicit about which handlers are default on.
Fix a couple of typos.
Expect the user to submit valid AccessCode.
Spec up exported functions from erollbar_encoder.
There is no such thing as filter anymore.
Spec up the messages that will pass through handlers,
and add it into the handler documentation.
Merge pull request #18 from omarkj/omarkj-support-arbitrary-errors

Support Arbitrary Errors