Make model identifier (version code) string public #45

Hi @Ekhoo, thanks for this library, I like it and use it often.

Sometimes model identifier is used in a "raw" value (i.e. "iPhone9,4"), for instance when sending client device information to a backend or similar, but in current version of Device this info is hidden behind static fileprivate func getVersionCode() -> String. Would you mind making that public?

Also, is there a particular reason for all of these version, size, type to be static methods instead of static properties? To me it seems a little bit more convenient if those were just properties instead of methods... What do you think?

Cheers,
// T