rfd plugin with unittests, modified fuzzer.py

rfd.py - Reflected FIle Download plugin
test_rfd.py - unittests for it
fuzzer.py - was modified so that it is possible to choose mutants with mutant_tuple kward of the create_mutants method
It's not a good idea to add a new kwarg before an existing one. In python you can do this: `create_mutants(freq, [], False)` , which will assign False to `append`. If I ever wrote some code like that, now I won't work because it will try to assign False to mutant_tuple.

I would add the new kwarg always at the end of the list of arguments.