[WIP]metrics for lua bouncer

[sabban]

This adds the metrics feature to the lua bouncer:
It creates a timer thread with the ngx.timer.at function and manages to send metrics with the help of a new "plugins" metrics lib that manages the metrics.

There are still some open questions:

• How to fill the os.name and os.version fields ?
• taking care of the last_pull field ? (What is it for, by the way ?)
• there are still a lot of debug entries marked with --debug for testing purposes to be cleaned before merging.