Consider using stdlib's testing's executable examples instead of a standalone example/main.go

[josharian]

This has a few advantages:

• It helps prevent accidental breakages like #10, particularly with help from CI such as travis.
• It integrates nicely with godoc.
• Having the expected output in the example helps the reader. (Had there been expected output in main.go, I would very likely have filed a PR to fix #10 instead of filing it as an issue, but I don't know what the intention of the example is.)
• When others vendor your library, go build ./... doesn't accidentally generate unneeded executables.

[tylerstillwater]

I agree. We should do this, @matryer