Re-layout so the installed binary has a better name.

Did a bit of digging around on how Go binaries and packages are usually done, and this seems to be fairly idiomatic (at least in usage, the "internal" sub-package could be differently named). With this pull request merged in, ```go get github.com/google/ct-hackday-schwag``` will get you a ```ct-hackday-schwag``` binary installed somewhere in a ```bin``` of your $GOPATH.