fontinfo.plist import/export v4. update for PR 215

see https://github.com/metapolator/metapolator/pull/215
If we haven't imported them are we planning to try to reference the source UFOs when we export an instance in order to get the metadata at that stage? It seems that when you setup a master with the source UFO available, that is the optimal time to grab the fontinfo.plist information too? If we are going to do some form of numeric interpolation to create some of the fontinfo.plist files for instances then I think we are going to want to be able to access the original metadata from the imported masters.