add log and exception support to logger

[jrhosk]

See title.

[jrhosk]

This adds two additional logging options:

• logger.log() which logs according to the current logging level returned by logger.level
• logger.exception which logger at the error level but adds the exception information.

One additional things, logger.get_logger() now returns the logger specified in $VIPER_LOGGER_NAME or a default logger 'graphviper' when no argument is given.