[datafeed] add parser to import cubicweb xml
this parser is configurable through a mapping, which basically
tells which entity's relation should be copied. Multiple actions
to control backport of data are available:
* 'copy': copy the external entities locally (though marked as
coming from the data feed source)
* 'link': find similar entity internaly (e.g. for states)
* 'link-or-create': try to find a similar entity internaly, and create
it if necessary (won't be marked as coming from the data feed source,
e.g. for tags)