XML serialization capability

Add a method, as_xml, which returns an string pointer containing
XML representing this TBX-Min document. Test roundtrip reading/writing.

Change basic.tbx to reflect the current name of the dialect (TBX-Min)
so that it matches with the output.
added doc_lang attribute to Min

Store the document language and make is accessible via doc_lang. This
appears in the XML root as an xml:lang attribute, and indicates the
language used for outside of langGroup elements, such as in the title
or origin elements.
added TODO

Add a project TODO file to keep track of future tasks.