fixed #53: rst: correct namespace edge attributes for edges between segments
Merge branch 'dev'
Merge branch 'dev'
added etree element generation to SaltLabel class
outsourced get_xsi_type to salt.util.py
refactored imports; refactored get_layer_id() to get_layer_ids()
refactored SaltElement. init builds from primitives; from_etree builds from etree element
UNFINISHED: refactored SaltEdge. init builds from primitives, from_etree() from etree element