Improve documentation to clear lint errors

[HarryR]

Additionally generate documentation using GoDoc - https://github.com/golang/gddo

./curvepoint.go:25:1: exported method CurvePoint.MarshalJSON should have comment or be unexported
./curvepoint.go:37:1: exported method CurvePoint.UnmarshalJSON should have comment or be unexported
./curvepoint.go:65:1: exported method CurvePoint.Prime should have comment or be unexported
./curvepoint.go:69:1: exported method CurvePoint.Order should have comment or be unexported
./curvepoint.go:106:1: exported method CurvePoint.GetXY should have comment or be unexported
./curvepoint.go:118:1: exported method CurvePoint.SetFromXY should have comment or be unexported
./curvepoint.go:137:1: exported method CurvePoint.Marshal should have comment or be unexported
./curvepoint.go:141:1: exported method CurvePoint.Unmarshal should have comment or be unexported
./curvepoint.go:155:1: exported function NewCurvePointFromString should have comment or be unexported

./helpers.go:11:1: exported function IsDigit should have comment or be unexported
./main.go:103:3: don't use underscores in Go names; var keys_file should be keysFile
./ring.go:20:1: exported method Ring.MarshalJSON should have comment or be unexported
./ring.go:35:1: exported method Ring.UnmarshalJSON should have comment or be unexported
./ring.go:65:1: exported method Ring.PublicKeysHashed should have comment or be unexported

./ringsignature.go:19:1: exported method RingSignature.MarshalJSON should have comment or be unexported
./ringsignature.go:35:1: exported method RingSignature.UnmarshalJSON should have comment or be unexported