Make MrsVariable sortable.
Make EPs and Nodes sortable, remove sorted_eps()

Similar to MrsVariables, EPs and Nodes are now sortable, based on:
 1. cfrom
 2. cto
 3. -is_quantifier() (meaning quantifiers come before quantifiees)
 4. pred.lemma (just as a fallback)

Now that we can just do sorted(eps), the sorted_eps() function is removed from pyDelphin
Fix a regression in labelset_head()

Introducing IVs to the graph means the minimum out_degree for normal nodes is no longer 0, but 1. This was causing too many heads to be reported, and in the wrong order.
Fix up SimpleDmrs output.
Add exporters with D3 output.
Add preliminary tikz exporter.
Fix conflict with exporters.