python: add journal backend for the logging framework

Supports Python versions 2.6 through 3.3 (tested on 2.7 and 3.2).
See JournalHandler docstring for usage details.

[zj: - use send() instead of using sendv() directly
     - do exception handling like in the logging module
     - bumped min version to python2.6, since the module
       does not work with python2.5 anyway ]