Introduction of basic logging

Hi,

First step towards [#156](emoncms/emoncms#156) - this introduces a new logging module which wraps the log4php implementation, and adds a couple of log calls which I used to help me debug a test script and SwiftMailer failures.

The log configuration will be deployed to /etc/conf/ as part of debian installation. It's backwards compatible with previous installations though, as it silently handles a lack of log4php on the system.