Adds the ability to remove channels as request in [Issue #2]

In order to add the ability to remove channels, as requested in **issue** #2, there was a bit of refactoring of the `Antenna` class. Each logging medium now has a `name` parameter that is used as the `channels` dictionary **key**. 
When channel is added or removed a notification is broadcasted with the **name** of the log channel as value in the `userInfo` dictionary.

I also added unit test for adding/removing channel.