Add interface to export and import XTM

[ajenhl]

EATSML provides a serialisation format of EATS data that is relatively easy to understand and use in client tools. It is not at all useful as a general format for the data, for use in non-EATS contexts. An XTM export (and, for completeness, import) would be handy.

On the EATS side, this requires only connecting an interface to the underlying django-tmapi functionality. As such, this issue depends on adding this functionality to django-tmapi (ajenhl/django-tmapi#1).