Add SEL (system event log) support

[korekhov]

Please, add SEL support into rubyipmi - useful monitoring and troubleshooting tool.

These are supported subcommands in ipmitool, but not all of them required, IMHO:

SEL Commands: info clear delete list elist get add time save readraw writeraw interpret

I think, these RO command must be exposed: info, list, elist, get , time

Plus, "clear", "delete " and "time " as RW ones.

The manpage is missing the description of "interpret" subcommand though :(