refactored svg vs. png handling code to address Issue #162. bug not fixed yet, but this is progress.
everything is on track to fix issue #162. committing now just before renaming pathway-svg to pathvisiojs-diagram
refactoring complete - issue #162 appears fixed.
minor update - add back signal to tests that diagram has finished loading. everything is now working. tests all pass.
works for everything except groups
Issue #162 is now fixed, and Alex's code from Commit 09c0810b577dcd3be8c3b143b5f0255f7b39dc89 is merged with this. All tests pass.