Remove unused code and further implement `def` command

This commit removes some old code (notably `parse_user` which serves no
purpose with the creation of `IRCPacket`) and implements the basic
functionality of the `def` command back into the threading branch. After
the `def` command is fully implemented again and bugs are hammered out,
this branch (threading) will be merged into the mainline probot source
code.

Future work will be on code cleanups and improving the program's
structure.