Ability to move SqlFu-specific attributes from POCO's to their metadata buddy classes

I don't like decorating my POCO's with data-access-specific attributes, and one way to keep them agnostic is by adding a metadata buddy class using the [MetadataType] attribute and applying all class- and member-level attributes (validation, data-access-related, etc.) in the buddy class. In order for these metadata attributes to be recognized by TableInfo.cs I have added an enhanced version of your GetSingleAttribute<T> extension method that checks the metadata buddy class (if one exists) for an attribute if it fails to find it in the main class. Here's this enhanced method. I will also add corresponding changes to the TableInfo.cs class.