wip: AnnotationGraph inherits from nx.MultiDiGraph
wip: T{Anchored|Floating|Start|End} classes
Merge tag '0.1.6' into develop

Version 0.1.6
Merge tag '0.1.6.1' into develop

Version 0.1.6.1
Merge tag '0.1.6.2' into develop

Version 0.1.6.2
feat: defaults is to not overwrite existing files (use --force option)
fix: bug in T.__eq__
feat: methods 'floating' and 'anchored' to get corresponding nodes
doc: AnnotationGraph and T{Floating|Anchored|...}
feat: JSON input/output
Merge branch 'feature/annotation_graph' into develop
feat: tvd.create www mode uses new JSON io
fix: re-init TFloating label generator before each resource
feat: AnnotationGraph.{save|load}
chore: tvd.create makes use of AnnotationGraph.save()
Merge branch 'release/0.1.7'
fix: requests 2.3.0 is not yet pip-installabel
Merge branch 'hotfix/0.1.8'