lua-factory: Support fetching multiple URLs

https://bugzilla.gnome.org/show_bug.cgi?id=725188
lua-factory: Don't require a source description

The source description isn't usually used in front-ends, and
isn't required for C sources, so don't require it for Lua ones.
guardian-videos: Add implementation in Lua

To replace the old C one. The main difference is not the language
though, but the service used to gather the list of videos, which has
more items, and results with more metadata.

https://bugzilla.gnome.org/show_bug.cgi?id=724644
guardian-videos: Remove C plugin

It is obsoleted by the same plugin implemented in lua.

This reverts commits 79635d20212c431142487dbb51cbcc96a5cfe587
and fa49c5a92f7ba00d3b74f148e401fc7ee81d852d.

https://bugzilla.gnome.org/show_bug.cgi?id=724644
build: Required Grilo >= 0.2.9

Due the new source-tags property.