Added logging support using the standard python module.

1. Added command line options for selecting a log file and setting
a log level. ERROR level is used by default, -vvv selects DEBUG log level.

2. Updated logger.Logger class so that it can be used with
logging.StreamHandler

3. Set VTGUI.logger as stream handler for logging.

4. Added build directory to gitignore.
Merge pull request #17 from alexey-naydenov/logfile

Added logging support using the standard python module.