opensubtitles: Add plugin

Add a plugin that will get a list of available subtitles available
for a video, given its gibest hash.

https://bugzilla.gnome.org/show_bug.cgi?id=740871
podcasts: Fix crasher when DB could not be opened

Don't try to close the database if it is invalid.

https://bugzilla.gnome.org/show_bug.cgi?id=741258