Add a message handler to handle messages

[lc-at]

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[reinier-millo]

I have added to develop branch the binary reader for WhatsApp messages. Currently the implementation is receiving text plain messages. To integrate with the message handler i have added a message callback. Changes are uploaded with #21.

There is some work to do with the received message to identify different type of messages. Currently media messages are not decoding successfully. Decoding is failing with the protobuf decode. I'm looking some references to identify how to solve it and allow to receive media messages. Currently media messages are received but don't decoded and identified successfully.