Added IdPropertyFinder and related unit tests
#4 Implemented ForeignKeyInitializer hook.
#4 Stopped attempting to read Id of null references.
#4 Removed casting on assignment.
Merge pull request #5 from mikezhuyuan/ft-idpropertyfinder

Added IdPropertyFinder to allow finding Id property configureable
Fixed #6
Updated readme.
Caching id property in IdPropertyFinder
More unit tests for IdPropertyFinderFacts.
More meaningful error msg in FakeDb.Db.MapId