gluon.contrib.estimator used a global Logger obtained via
`logging.getLogger('gluon.contrib.estimator.event_handlers')`. This logger used
to be configured every time a gluon.contrib.estimator.LoggingHandler was
created, which is a bug. We can't modify a global Logger instance whenever the
user creates an Estimator and a LoggingHandler.

Instead, this commit separates the LoggingHandler (responsible for logging
metadata during estimator.fit) from the configuration of the Logger.

We expose the Logger as attribute of the Estimator, and configure it to output
to stdout by default. Instructions are given how users can configure the
Estimator.logger to log to a file instead.