Visualize blockchains and executions in browser

[pkel]

The current approach to visualization is to generate GraphViz dot files and render them to PNG. This works for small blockchains and short executions. It also works on the local machine only.

A better approach might be to log executions into a graph exchange format (e.g. graphml), then render it in a browser using javascript. Candidate libraries are sigma.js and cytoscape.js. It would be really nice if make visualize would create a website listing all available executions. I could also support loading an execution that is not listed, e.g. from a raised exception on a different machine.

Tasks:

• Log executions into graph exchange format (see 2c368cb)
• Visualize in browser