Randomize time of execution for optimize script.

By default, time will be between midnight and 7:00 any day of the week.

The values are currently hard-coded. This can have serious I/O
repercussions on a server with multiple VMs that run mysql since they
will all run their optimizations simultaneously.
document the new arguments for setting optimize run time
Fix munin plugin class on Debian
Fix command output capturing in optimize_tables.rb for Ruby 1.9
Merge branch 'randomize_optimize'