22 add XI-IATI

Adds XI-IATI as a registration agency. This should be ready to merge, but then there will need to:

Update the non-embedded changelog text: https://github.com/IATI/IATI-Guidance/blob/master/en/upgrades/nonembedded-codelist-changelog.rst
Update the tests (tests is currently on it's own branch)

AND it's still unclear as to when this merge should happen in relation to the release of 2.01 which is due on 16th Oct 2014