Add initialisation for existing serial port

[eblot]

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.