minor: made conll.py executable
conll: added coreferences to CoNLL2009 exporter
added "make conll" directive
added header/footer to conll output
conll: added self.tok2chain (cf. issue #50)
fixes #50 and closes #49: conll export now contains proper coreference chains
fixes #51: conll exports singletons as well
flake8: conll.py
minor: flake8 stuff
fixed get_annotation_layers(): result set now includes edge AND node layers
added select_edges_by_layer()
added select_edges_by(), which filters edge type and layer
refactored select_edges_by(); removed select_edges_by_... functions
flake8: discoursegraph.py
added ns, name, root, sentences and tokens to discoursegraphs base class
closes #24: fixed self.name assignments in importer classes
made .reverse() possible for anaphoricity and rst