Use dagre v0.3.0

The new version of dagre takes a dagre.Digraph as input, instead of
node and edge maps. In this case, the resulting plugin is about 30 lines
shorter and the logic is simpler.

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A few notes:

1. I preserved checks that the node and edge ids were not already used
in the graph. It's unclear when that would happen, but I decided to err
on the side of caution.

2. I use the name dagreGraph, inputGraph, and layoutGraph in the updated
layout and _prepareData functions. These names may be too close to the
name 'graph' used as input to layout. I'm happy to change these if you
have a suggestion.

3. This change only includes changes to the plugin source. It looks like
the dist is checked in. I can run a target to regenerate the files
(grunt all?) and include them in this patch if it is your preference.

4. I noticed that using setLinkVertices = true yields edges that are contorted
in odd ways. I didn't look too closely, but if this is something that needs
attention, let me know. I confirmed that the same occurred pre-patch.