Allow configuring log_on_* operator

The default operator for log_on_success and log_on_failure settings is
+=, which means that you cannot reduce the amount of logging.  This
change allows specifying arbitrary operators but does not change the
default.
Merge pull request #35 from dvorak/log_on_operators

Allow configuring log_on_* operator
Prep for 1.3.0 release.
Merge pull request #36 from mhaskel/1.3.0-prep

Prep for 1.3.0 release.
Prepare 1.3.1 release.
Merge pull request #38 from apenney/131-release

Prepare 1.3.1 release.