Consider opening CBM classes for subclassing

[s-hocking]

The CBM library is great for simulating a full Bluetooth connection workflow, but sometimes I just want to satisfy a dependency on a CoreBluetooth object in order to test other parts of a class. For example, I might have a class with a dependency on a CBPeripheral object that I want to test without using any CoreBluetooth capabilities.

To achieve this use case it would be helpful if the core CBM classes were open, so that they could be subclassed and used this way.

Subclassing might also be a useful way of customising CBM behaviour for test purposes.

[philips77]

If you could prepare PR, I'll release the version.