POCO LINQ Update

Here is my long-promised near-POCO solution for LINQ to SQL.

As planned, it is deliberately not 100% pure POCO, but it does implement these key goals:

1. Entity classes are simple enough to be hand-written (no need to generate them with SQL Metal or VS 2008)
2. Mapping is easy (no need to manually map every property via attributes or external XML)
3. Lazy-loading works
4. Bi-directional relationships work (update one end, and the other is synchronized automatically)