In my example plugin, I only needed a few functions exported to theplugins to implement the BBC News plugin:
https://mail.gnome.org/archives/grilo-list/2012-July/msg00021.html
I would include a function to download a URL as text, a callback to send
out media information, a way to return an error, and some XML/XPath and
Json helpers (using libxml2 and json-glib).
If particular plugins require more libraries (say, MD5 sums generation,
etc.) we should make it easy to add new functions to grilo's builtin Lua
functions.