Add more public API features

[kangz12345]

New features:

• appNames(): All the names of the existing apps
• channelNames(): All the names of the existing channels
• subscriberNames(channel): All the subscriber app names of the channel
• subscribe(app, channel): The app subscribes to the channel
• unsubscribe(app, channel): The app unsubscribes from the channel

This closes #151.

[BECATRUE]

Wow! TODO causes a Pylint warning, but I think it doesn't matter.

[kangz12345]

@BECATRUE Please approve this again, since I modified the code to remove the pylint warning.