Added option to download submission with tests and nbproject folder

So that its easier for TA's to debug students code / try it on their
machines.
Suggest following method comment:

    # Packages a submitted ZIP of the given exercise into a TAR or a ZIP with tests
    # added from the clone (to ensure they haven't been tampered with).
    #
    # - zip_path must be the path to the submitted zip.
    # - return_file_path is the path to the file to be generated.
    # - extra_params are written as shell export statements into into .tmcparams.
    # - config options:
    #   - tests_from_stub: includes tests from the stub instead of the clone.
    #                      This effectively excludes hidden tests.
    #   - no_tmc_run: does not include the tmc-run file
    #   - format: may be set to :zip to get a zip file. Defaults to :tar.