Modified step in behave's step.py.

The step that invokes backup without parameters actually added the -vv
parameter. Now the verbosity parameter must be specified explicitely.
Moved logging config in _logging.

I changed my mind. I will be using the _logging module again. I think it
is simpler since both modules config and contoller do stuff with
logging.
Made the definition of the _logger attribute DRY.

Most classes in this program define a _logger attribute.
This is now done in a Logging class of the _logging module. Classes must
simply subclass it and start using methods of self._logger.

This makes future changes to the loggers naming scheme a simple matter
of modifying the Logging class.
If backup exits with a non-zero return code, logging will reflect this.
Added support for the --dry-run option in modules engine and config.
Snapshot is now a class factory.

The factory adds a new keyword argument to the Snapshot callable.
If dry_run==True, Methods that modify the filesystem will be replaced by
"lambda *args: None".

Actually, I just wanted to save this idea for another occasion. I'm
going to go for an inherited class and decorators instead because I have
many classes to dry_run enable.
Revert "Snapshot is now a class factory."

This reverts commit 17cb2066f444ffdb1101c2baba17fc17b1620090.
Created the dry_run module: tools for supporting the dry_run operation.