Use "go generate" for bsongen instead of Makefile.

Now the bsongen command can live next to the type it operates on.

Note that you need Go 1.4 to run "go generate", but that only needs
to be done when you've modified a type that is the subject of a
generate command.
Merge pull request #277 from youtube/go-generate

Use "go generate" for bsongen instead of Makefile.