Clean up and document log.py.

This also removes module-level use of "g" or imports of r2.lib.amqp so
that importing this module is possible before the app is fully set up.
Use WebError Reporter objects for exception logging.

This brings log_q reporting and logging-based reporting (syslog)
together into one place as WebOb error reporters.
wrap-job: Don't cd into r2/r2/ when we just want to be in r2/.

This was causing Pygments to fail to import correctly in jobs since
r2/r2/ has a commands.py which was getting relatively imported by
accident. Yuck.