Fix and features

Hello,
i'm here again (i like so much your library), i've fixed some bugs,
plus i've added the support for nullable enum in the SelectGenerator.
Finally i've separated the default format for the unknwn translations, i've put it in the DefaultCulture, i think it's a good place.
I did it so i can call it in the SelectGenerator, extracting that logic from the LocalizedModelMetadataProvider.

I have slowness issues, so i've implemented a CachedStringProvider, but the first time it get the strings it still remain slow.
I will try to let LocalizedModelMetadataProvider extends CachedDataAnnotationsModelMetadataProvider to see if i gain some speed, but this imply to target the project to the framework 4.5