Fix #700: Tee is missing the writelines() method.
Add a fake isatty() method to Tee.
Added flag '--cover-nostream' to cover.py plugin

Added a new flag '--cover-nostream' to the cover.py plugin.  This flag will
prevent a final coverage report from being pushed to the stdout/stderr stream
when nosetests is run with coverage enabled.  The flag does not suppress the
other output methods (HTML, XML).