Improve Rawrbot's social skills

[pfaffle]

The Social plugin is currently pretty minimal and could be expanded. Some potential features to add include:

• Ability to recognize more greetings/goodbyes.
• Additional variety in responses.
• Ability to recognize other kinds of common small talk questions and respond to them.

This might require some additional intelligence in this plugin and/or the Learning plugin in order to better recognize which one of them should be triggered. It's also possible that both plugins might need to be merged.