Include user issuing command to the plugin functions

[KDercksen]

Right now, plugin commands only receive the message part of PRIVMSG lines. For example with the karma command:

user | something++

The plus_karma function only receives 'something' but doesn't know from what user that command came. Now for the karma plugin this isn't really relevant, but for future plugins (for example the ~tell proposed in #6) it is necessary that this information is available.

I propose that the standard arguments for every plugin function change from (self, match) to (self, user, match) so the user info is available might it be needed.