Schema: Add "dump_as" parameter (plus tests)

also improve docstrings, comments
Rename Schema-attr dump_as to _dump_as
Update CHANGELOG.
Implement enum to specify marshalling format ...

including tests & docstrings, ...
Remove support for dumping lists and lists of tuples

these features seemed unnecessary and bloated on second thought.

What remains is the possibility to declare a Schema instance as
"ordered" which results in the instance outputting OrderedDict objects
instead of simple dict objects.
Fix schemaobj._dump_function not having access to globals.

This enables _dump_function to reference OrderedDict for output of
ordered dicts (didn't work before this commit).

Also: extend test a little
Update CHANGELOG.
Merge branch 'feature/dump-ordered' into develop