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.
logging: allow to set different logger for param, default is root logger.
Merge branch 'master' into logging
Merge branch 'master' into logging
Merge branch 'master' into logging
Merge branch 'master' into logging
logging: Undo removal of verbose() function

Readd verbose() function for backwards-compatibility. Note that there is
no logging-level in the logging module that corresponds to the old
VERBOSE logging level; instead verbose() now logs to logging.DEBUG.
logging: Add custom logging-level VERBOSE

This commit fixes verbose() using the DEBUG logging-level, and instead
uses a custom logging level VERBOSE (INFO-1). This logging level is
defined in param.parameterized. To provide all logging-levels in one
module namespace, the logging-levels defined in the logging module are
imported into the param.parameterized namespace.

The following example shows how to initialize the root-logger with the
VERBOSE log-level and a custom log-format:
  import logging
  import param.parameterized
  logging.basicConfig(level=param.parameterized.VERBOSE, format='[%(levelname)s] %(message)s')