Welcome to openHAB, then. Nice intro, most people want us to do something for them 
On the organizational part of contributing, see http://docs.openhab.org/appendix/contributing.html.
On HVAC concepts… well.OH is a framework in that it provides access to devices on an abstract layer. There’s different types of items and groups, but essentially this is rather generic and there’s no HVAC specific types.
To speak frankly, there is no such thing as a OH-builtin HVAC concept (nor is there any ‘occupancy’ concept).
In OH, users (and not the core or binding developers) are expected to implement these concepts on their own, typically using the OH rules engine. That is, your heating/cooling main control loop is to happen outside of the binding.
So I would recommend you to go for a two-stage implementation: an OH binding which is to translate/expose device-provided functions of your thermostats (in)to the abstract OH layer, and a set of OH rules to show a sample implementation how a user could make use of the binding.
What is available as part of OH is an active community that encourages and helps in sharing rules, from snippets to full example implementation of concepts.
There is a first set of HVAC control implementations to work with specific devices, but no comprehensive concept (which I believe there never will be as specifically on HVAC, there’s so many devices and concepts that are incompatible with each other).
Same goes for presence (or ‘occupancy’). This is another concept sitting next to HVAC control. There are users that already have implemented this. I’d recommend to assume that there is an item ‘presence’ or eventually an item per-room that gets ‘magically’ set, either by some automatism, or by having the user manually switch it via UI.
Check out the tutorials section and use the search function on this forum to find examples on rules.
Here’s a good thread with a sample implementation, including discussion on presence.