Only use bundlewrap's handler while logging.

When calling `logging.basicConfig`, a first handler is established,
usually with the default format (`logging.BASIC_FORMAT`).

The format was overridden, but the handler still exists. Now by
attaching bw's handler to the root logger, exactly one handler
should be used throughout bw.
Remove second addition of handler as it is already attached to
the root logger.
Merge pull request #133 from mfriedenhagen/issue-130

Only use bundlewrap's handler while logging. (#130)