Using idiomatic file naming conventions
Remove redundant float syntax

While I was here, I also changed the order of conditions in FW so that
cheap conditions are checked first.
Use a var for Inf
TODO for package-level comment
Simplify parameter names
Resolve missed conflict and fix incorrect test

I have also highlighted the difference between the directed and
undirected tests.
Fix more missed conflicts
Move inf into util

Leaving concrete.go for overall docs. Should rename to doc.go.
Mark that NullHeuristic ignores input nodes
Simplify interface parameters
Improve error messages for oversize graphs
Simplify receiver name

This was done previously but clobbered in a bad local rebase.
Improve documentation comments
Fix bugs found while working on graph/set changes

This will be a significant argument for the comming change list; that is
that the keys should be int rather than interface{}. These three bugs
were found because of that change.
Add Degree method for undirected graph
Remove unnecessary blank identifiers
Guard maxID decrement

Updates issue #20.
Elide unused name
Test for maxID guard

Fixes issue #20.
Remove another math.Inf call