Add initialisation for existing serial port

It would be great if BlueGigaAPI could be instanciated from an existing Serial instance:

```
class BlueGigaAPI(object):
    def __init__(self, port, callbacks=None, baud=115200, timeout=1):
```

This would give the opportunity to use `serial.serial_for_url()`, customize features (such as HW flow control with RTS/CTS), perform pre-initialisation communication, etc.
