Escape printf-command characters in log messages

Replace "%" with "%%" in log messages prior to passing to Printf to
prevent output like "%!B(MISSING)" from appearing instead of "%2B".

Signed-off-by: John Tuley <jtuley@pivotallabs.com>
slightly simpler log escaping
Merge pull request #44 from cloudfoundry/escape-log-messages

Escape printf-command characters in log messages