Add NewPageFilter option to include redirects
Fix NamespaceFilter

- Since namespace is filtered before filters are applied, the effect
  of the NamespaceFilter class should be nothing. Since filters work
  in parallel now, it must return an empty list.
Assume homesite prefix for PageFilter if no prefix given
Cleanup
Let BacklinkFilter ignore non-existing pages

Especially when checking multiple wikis, pages might not
exist with the same name on all the wikis.
Update uploadplot to work with latest mwclient
Use rollbar for error logging