Build arm32 binaries

[ggreif]

In #11 I made an attempt to build for Raspberry 2/3 (which are 32 bit). Turns out env GOOS=linux GOARCH=arm go build creates the right binary. This has the advantage that it runs on the original raspi too.

So adding a step to build for 32-bit ARM should be a matter of a few lines.

All the Pi owners are anxiously awaiting keysmith-linux-arm32.tar.gz!