Bluetooth Music Controller for iPhone/ HTC / Samsung etc smartphones, and iPods

We are a group of mechanical engineering students and would like to create a Bluetooth music controller, similar to a "Satechi BT MediaRemote". However, we would only like to have the functionality of play/pause, skip forward/back, and volume control. The area we are struggling with is the electronics and phone interaction side of things. We require this system to be compact and as inexpensive as is feasible. Currently the project is in the prototyping stage. Ideally we would like a complete design for the electrical system which would be able to integrate with as wide a range of devices listed above as is feasible. There are already systems out there which achieve this, so the work should be nothing too new and it may just be a case of sourcing a complete system. We view the electrical system as: battery (