Remove language testing from on_task_download

If plugin receives a chapter page, assume its language has already
been checked, either by the urlrewriter or language setting on rss
feed.
Ensure rss url has language filter if filtering

If plugin is filtering by language, ensure rss is requesting a filtered
feed (if possible). Also changes format of self.lang to languages
separated by %3B (urlencoded semicolon) to ease usage and changes from
using urlparse.urlparse to the recommended urlparse.urlsplit.