Several changes to mediator.py

1) Fixed several methods whose parameters were not being used (setTm in
TC, Reporter, and DB), and places where class members were being
assigned the value of global variables.
2) Moved an import to the top of the file, as suggested in pep8
(http://www.python.org/dev/peps/pep-0008/#imports).
3) Removed an unused local variable in TestManager's publishReport
method.