Move replace_extension helper function to sickrage and add tests

sickbeard/helpers.py

sickbeard/metadata/generic.py

sickbeard/metadata/kodi.py

sickbeard/metadata/mede8er.py

sickbeard/metadata/mediabrowser.py

sickbeard/metadata/wdtv.py

sickbeard/postProcessor.py

sickbeard/tv.py

sickrage/helper/common.py

tests/sickrage_tests/helper/common_tests.py