One item can have multiple media:content elements which may be located
in multiple media:group element to indicate that content is the same but
for the format.
This patch adds the ability to add multiple content elements and define
the group to which they go belong.
If no group is specified, all elements are located in a `default` group.
If the group is set to None, the content element is directly attached to
the item element.
Part of #58
Using `fe.media.media_content(…)` is quite redundant. iThis patch
changes the name to `fe.media.content(…)`. The same goes for the
thumbnail method.
Part of #58
