update megaraid_serial fact to deal with SM motherboards

Per #29, the LSI controller on at least some models of SuperMicro
motherboards does not report a serial number.  The megaraid_serial fact
has not gracefully handling this case and facter was generating errors.
add megacli output example provided in issue #29
Merge pull request #30 from jhoblitt/feature/megaraid_facts

update megaraid_serial fact to deal with SM motherboards