Take an instance rather than class

Measurements should be able to take an instanced class so that the
resulting report can be shared between several Measurements.
Add postProcessor support to NetTest

If a NetTestCase has multiple test_ methods, then the report write shall
not occur until all of the individual Measurements have completed.

This also changes the postProcessor API slightly; postProcessors are now
expected to return a report instance. If a postProcessor is not
supplied, then the report object from the instanced NetTestCase class is
passed to Report.write()
Check to see if setUp was already called correctly

The report dict is already created in the constructor, so we look to see
if instead the input argument was already set.
Merge pull request #211 from TheTorProject/feature/support_postprocessor_api

Feature/support postprocessor api