Update README.md

added Ian to authors, overdue. Thanks Ian!
Simplify the producer viz code. Don't try do anything clever, show every node in the graph even IdentityKeyedProducers
Add's test for failing user graph, where merge node was also a fork node.

Adds fix also
Minor variable name cleanup
Add some more info in debug output in test
In light of the not passing the maps around, the rest of this code gets much simplified too.
Merge pull request #417 from twitter/feature/fix_merge_planner_bug

Feature/fix merge planner bug
Merge branch 'develop' of github.com:twitter/summingbird into feature/shard-FM
Use the latest scalding's shard function