Make the event destinations pluggable.

Should be completely backwards compatible. However, you can now add objects to the #endpoints array that respond to #log_event to send events to other recipients. E.g., another interesting endpoint could be StatsD.

I removed the unstructured #log call, because it doesn't really work well in this concept, and I don't think it belongs here in the first place. If you want to log an unstructured message, you'd just use Rails.logger.log.
Move colour definitions to appropriate class.
Clean up json endpoint.
Code review changes.
Update README with API changes.
Include a newline when writing to $stderr.
Remove constructor helpers.
Merge pull request #7 from Shopify/pluggable_endpoints

Make the event destinations pluggable.