Journal plugin

Introduce test journal plugin, that keeps a record of status changes to the tests during a job. Each one of the status changes can then be sent to an avocado server to give feedback to the test owner.

Activate it with the ```--journal``` command line option. The journal will be created on the results directory with the ```.journal.sqlite``` name. This will be read by the (upcoming) journal replay tool.

Please note that this plugin makes the need for multiple output plugins more urgent.