[buteo-syncfw] Make logging configurable at runtime with MSYNCD_LOGGING_LEVEL

Read the default logging level from a MSYNCD_LOGGING_LEVEL
environment variable. If it is not set, the level defaults to 4
(warning level).

Also remove the logging code that installs custom message handlers
as this doesn't play well with other processes that are logging.
Merge pull request #58 from nemomobile/logging-level-envvar

[buteo-syncfw] Make logging configurable at runtime with MSYNCD_LOGGING_...