added SaltXML importer from pcc_graphs repo
salt importer: changed path to test file
renamed salt.py to saltxmi.py
salt: added second test file (w/ layers)
refactoring: renamed SaltDiGraph to SaltXMIGraph
added layer names to __str__ of SaltDocument
salt: updated docs, added string2value() for hex conversion
refactoring: renamed Element to SaltElement
refactoring: renamed Edge to SaltEdge
refactoring: renamed Layer to SaltLayer
refactoring: renamed Node to SaltNode
oops! fixed variable names after refactoring.
added SaltLabel class; renamed string2value() to strin2xmihex()
Merge branch 'dev'