Fix for python3.8

Fix write_data after connection_close in python 3.8

Subscriptions stored in client

Client now stores it's subscriptions

• on_subscribe callback signature has changed, argument properties has beed added: on_subscribe(client, mid, qos, properties)
• client.subscriptions - list of subscriptions
• client.get_subscriptions_by_mid(mid) - get list of subscriptions with certain message identifier. May be useful in on_subscribe callback in order to handle (un-)successful subscription. Note that after on_subscribe is finished, mid is freed and can be used in other messages, so it's also removed from subscriptions.
• client.get_subscription_by_identifier(subscription_identifier) - get list of subscriptions with certain subscription identifier. May be useful in on_message callback.

Async on_message bug fixes

Bug fixes in async on_message callback. Now it works properly for partial coroutines and optimistic_acknowledgement=true clients

Fix disconnect packet in MQTT3.1.1

• fixed disconnect packet in MQTT3.1.1 This bug caused unexpected will messages coming after gentle disconnect
• added info about asynchronous on_message callback to README

Multiple topics subscriptions

• Multiple topics subscription added:

client.subscribe([gmqtt.Subscription('TEST/+', qos=1), gmqtt.Subscription('TEST', qos=0)],
                 subscription_identifier=1)

• Fixed resend QoS > 0 messages
• Added Client.is_connected property, which tells if connection is alive and CONNACK was received

Code improvements

Removed unnecessary link to current event loop in sync method and etc.

Speed up packets read

Optimized packets read and parse

Asynchronous on message callbacks

• Added support for asynchronous callback messages. Value returned from callback future will be used as reason code in PUBACK packet
• added method to update client's _reconnect flag

UTF-8 encoding in JSON messages

Do not force ASCII in json messsages, use utf-8 encoding

Allow to set a few user properties;

v0.2.1

Multiple user properties in publish (#27)