Add SysTimeModel

[luizzmizz]

I added a new Instance::SysTimeModel package in order to retrieve v$sys_time_model data, which is really useful for drawing database activity. As you can see, I'm pretty newbie on perl and I needed three commits :(, even though this is a clone of your sysstats package

[lausser]

This looks like a copy-paste-bug:
['server::instance::systimemodel::listsystimemodel',
'list-systimemodel', undef,
'convenience function which lists all statistics from v$syswaitclass' ],