Add a connection change receiver

The class that I've added listens to network connection changes.

For now, it only establishes connection to node(s) if none is present. This fixes a situation when the user enters Antox without being connected to the internet: the only way he/she is going to connect to the Tox network is by connecting to the internet and restarting the app.
Now, just the activation of the internet is required.

Also, this class can be extended to handle many connect/disconnect situations.