Add parent and parentauthors when inserting modules

If module B is derived from module A, in the epub, we should get a
"derived_from_uri" field which looks something like
http://cnx.org/contents/module_a_uuid@version
The ident hash of module A is extracted from the "derived_from_uri" and
is matched against the existing modules to get the module_ident and
authors fields.  The module_ident and authors are then used as the
"parent" and "parentauthors" fields in the module B.
Remove duplicate call to add_pending_resource
Merge pull request #10 from Connexions/derived-copies

Add parent and parentauthors when inserting modules