Improve logging

A good habit when publishing a library in Python is to allow users to select the log level.

We can use a global logger with the name duniterpy:

logger = logging.getLogger("duniterpy")

To allow a more fine logging configuration, library modules can log on the module logger.

To get the module logger, you have to do this:

logger = logging.getLogger(__name__)

Then use the logger instance to log everything:

logger.error("Syntax error")