rename "origin" to "creator"

Rename the "origin" field to "creator" to make the field's purpose
more clear. As the "origin" field is the equivalent of the UTX
"creator" field, it should hold the name of the person or organization
that created the glossary. "origin" was a confusing name, since it
could mean the source dictionary title or something similar.
rename "title" to "id"

Rename the "title" attribute to be "id" instead.
The document "title" was meant to be equivalent to the UTX dictionary
ID. This should be a string uniquely identifying the document.
Converting the file ID to "title" seemed sub-par, as titles and IDs are
very different. Although a title was nice to have, the ID can be used
as a title if needed (and a file description will be added in the
future).
remove "doc_lang"

Remove the "doc_lang" attribute from Min.pm, as UTX has no concept of
a document language besides source and target.
add "date_created" field, code cleanups

Add a "date_created" field to Min.pm. The input should be an ISO 8601
string, just like in UTX. In TBX-Min, represent this in the header as
<dateCreated>. Croak on bad input.

Make code more readable in a few places where problems were found.
note ISO standards used for language codes

Calling the language strings "abbreviations" was too broad. Specify
instead that ISO 639 and 3166 standard codes should be used.