PR: Secure MQTT Implementation

[brentru]

Ref. Issue: #5

Changes:

• mqtt_client now defaults to secure MQTT, port 8883 (TLS/SSL):
  def __init__(self, username, key, service_host='io.adafruit.com', secure=True):

• For users who wish to use an insecure connection (router/network troubles), we'll support that too! Just add a secure = false parameter when the client instance is created:
  client = MQTTClient(ADAFRUIT_IO_USERNAME, ADAFRUIT_IO_KEY, **secure=False**)

• Insecure client connections return a console warning, connect on port 1883.

• Modified MQTT example to show both secure and insecure client connections.