Created Resource files for ProtoAssociative and ProtoImperative

@Benglin  @ke-yu  I created resource files for these 2 projects and renamed the StringConstants.cs to StringConstantsOLD.cs. Please take a look.
I think @lukechurch meant to ask for this to be renamed to `StringConstants.old` file (and completely removed it from the `csproj`), instead of `StringConstantsOld.cs`.