bot.unload_extension: also allow events with no module
bot.unload_extension: also allow events with no module

It turns out that events created in an eval command also cause
the issue described in #1506.

Ensure that events we remove are part of a module as well.

Also performs minor comment maintenance
("x", "first y", "then z") -> ("x", "y", "z")

Harmon758/discord.py,rapptz/discord.py,Harmon758/discord.py,khazhyk/discord.py,Rapptz/discord.py,imayhaveborkedit/discord.py