IDPF/epubcheck — pull request #450 by rdeltour (2014-09-05 09:11:27+00:00)
Title: Remove check OPF_023

`OPF_023` was checking that there was only one `metadata` element in
the OPF, but:

* this is already checked by RelaxNG schemas
* the check is flawed since `collection`s also allow `metadata`
children

@santoch or @apond please review

(it may be cleaner to first apply #445 and rebase this branch against master afterwards, I can do it if you give the go ahead on the PR).