move passwd under System menu
passwd: add a 'system' checkbox
system user/passwd classes
change system passwd [incomplete!]
passwd: use exceptions
OnBoard::System::Process != ::Process
Passwd: "passwords do not match" exception
stub system passwd change
change Unix passwords
System::Command.send_command

and move logic from System::User::Passwd
lib/onboard/system/command.rb: minor, coding style
untested System::FS namespace
system passwd: handle ro filesystem gracefully
passwd: Controller: a TODO
fix typo in lib/onboard/system/command.rb
simplify error handling API; use in passwd.html
passwd: better error handling
version bump 2013.02