minor changes improving stability

This is an intermediate fix. There's a user record that never had it's database created. The first time it shows up in the changes feed the revision starts with 2-. Not sure why this is. But we loose this user record if we rely on CouchRest::Changes.created which checks for a revision starting with 1-.

* do not log the SystemExit after the fork
* log error classes alongside messages
* unify logging in retry_request_once
* debug now logs every step
* info has one message per user created
* warn only logs errors during the actions
* error only logs crashes

This version mainly improves the stability and should keep tapicero from dying too early.

* couchrest_changes 0.0.5 will resume if the connection died.
* lock yajl version to 1.1.0 - 1.2.0 requires mkmf on the server
* retry couch requests in case they fail
* log all crashes

Couch sometimes responds with 500 or so. Often this is temporary. Let's retry once and log the error instead of dying.

--run-once will exit once all current changes have been processed
--rerun will process all users again even those that were processed before
--overwrite-security will overwrite security for user databases even
  if it has been set before.