#113 Improve support for non-string ID columns

[tunger]

This is an attempt to improve support for non-string ID columns as stated in #113.

Main changes (which need to be discussed):

• Change Load signatures to use object
• [breaking] Seperate Load methods which load multiple documents to LoadMany. Change signature to object[] or params object[] because IEnumerable<object> doesn't work for any type.

Please let me know if this is something you approve, or if we should solve this another way.

[tunger]

Alternative would be to create overloads for common key-types like Guid, int, long, string.

[tunger]

I added tons of overloads for string, int, long and Guid as overloads for the various Load methods, and dropped object. Should be easier to use now.

[PaulStovell]

I think you found a really elegant solution here - nice one!