A stacktrace is printed when no user CB is provided in the constructor

[baldoalessandro]

How to reproduce

>>> import iottly_sdk
>>> sdk = iottly_sdk.IottlySDK("Wilson")
>>> sdk.start()
>>> Exception in thread receiver_t:
Traceback (most recent call last):
  File "/usr/lib/python3.5/threading.py", line 914, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.5/threading.py", line 862, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/local/lib/python3.5/dist-packages/iottly_sdk-1.3.0-py3.5.egg/iottly_sdk/iottly.py", line 486, in _receive_msgs_from_agent
    self._process_msg_from_agent(msg)
  File "/usr/local/lib/python3.5/dist-packages/iottly_sdk-1.3.0-py3.5.egg/iottly_sdk/iottly.py", line 506, in _process_msg_from_agent
    self._handle_signals_from_agent(msg['signal'])
  File "/usr/local/lib/python3.5/dist-packages/iottly_sdk-1.3.0-py3.5.egg/iottly_sdk/iottly.py", line 519, in _handle_signals_from_agent
    self._on_connection_status_changed_cb(status)
TypeError: 'NoneType' object is not callable

Cause

iottly-sdk-python/iottly_sdk/iottly.py

Lines 513 to 519 in 890a18b

	def _handle_signals_from_agent(self, signal):
	if 'agentstatus' in signal:
	status = signal['agentstatus'] # TODO validate status
	self._on_agent_status_changed_cb(status)
	elif 'connectionstatus' in signal:
	status = signal['connectionstatus'] # TODO validate status
	self._on_connection_status_changed_cb(status)

The invocations should be guarded like in

iottly-sdk-python/iottly_sdk/iottly.py

Lines 550 to 551 in 890a18b

	if self._on_agent_status_changed_cb:
	self._on_agent_status_changed_cb('started')

Thanks to @Oscar-Campo and @lupodellasleppa 🎖️