Changed logging system to use Python 'logging' module.

Most of the logging functionality is now delegated to the 'logging'
module; important changes as a result of this:
- Parameterized class instances no longer have individual logging
  levels ('print_level' param). Instead, the logging level is that of
  the root logger and should be set through the 'logging' module.
- There is no explicit default logging level, as this is also determined
  by the 'logging' module.
- Removal of the verbose() function, as there is no equivalent logging
  level in the 'logging' module.
- The default message prefix depend on how the root logger is set
  up. Any application wishing to use logging, should set this up
  through the 'logging' module.