Remove unused (and confusing) arguments
Log full exception from cmd_function() crashes
divide only if total isn't 0 in cpugraph

Closes #367
ellipsize text in tasklist widget

Closes #366
Fixes problems with focus when clicking windows

Conflicts:
	libqtile/manager.py

Closes #365

@tych0: small change to remove cmd_dummy and just use a noop function.