Better Specification/Implementation Conformity

Just added a few little tweaks to follow the `Dockerfile` [documentation](http://docs.docker.io/en/latest/use/builder/#format) and [implementation](https://github.com/dotcloud/docker/blob/v0.6.1/buildfile.go#L490-L502) more closely, namely:
- commands are technically case-insensitive
- only lines which begin with a # are truly comments
- commands are only commands at the beginning of the line (and on a similar note, all commands MUST have at least one argument, hence the safe matching of `\s` after commands)