AvahiBrowser: Made it work more event based.

Now it emit a gsignal on a new service of desired type
MainWindow: Set up the AvahiBrowser

The service discovery part should run as soon as possible, i.e. at the start of the application. For now the publishing of the service will be made manually from the script AvahiPublisher.py
Fixed class name
Sections: Add a new page (KeysFromNetworkSection) that will be used for data transmission over network.

This is a temporary scenario which now it isn't even implemented: it should allow to write a message (e.g. key fingerprint) into the text editor and then pressing 'Send' will send the data through network to another isntance of Geysigning (for start should be just a script that displays that data)
KeysFromNetworkSections: Simulate downloading a key

It still needs to get the keydata from the network.
Set up logging
MainWindow: Made it to always return True in verify_service as name is something user defined and won't match what we're expecting.
Minor fix in adding a tuple in add_discovered_service
KeysFromNetworkSection: added a button to clear text inside the TextView

This is a temporary use
Sections: Refactored names and deleted unnecessary code
Sections: Deleted temporary class. Modified design of GetKeySection
Merge branch 'develop' into gui

Conflicts:
	Sections.py