Keeping track of spec evolution

The reference schema is *imml.rnc* (which loads *imml_kit.rnc*). Both **rnc** and **rng** flavors reside in *data/*.
To validate against your xml files, use **jing**. Exemples of valid xml files can be found in *test/fixtures/*: for instance, `jing -c ../../data/imml.rnc sales_report_2013-12_from_3025596562609.xml` should show no error.