Omit JFIF header by default

Do not create JFIF header (APP0 block) by default. This saves 18 bytes per image. Fixes issue #18. Unlike original PR, this provides scope to fallback to default behaviour. Not the tidiest patch... I would prefer (once solution is field tested) to strip out all related code, for tidier code base. Utility of APP14 (Adobe) marker should also be investigated in future- is it actually useful for Colorspace data in context of web?