InvalidCastException when DAO property doesn't match SQL column data type

FastDAO fails to warn you if a C# property definition doesn't match the SQL definition.

Common scenario:
1. Define column as `DATETIME NULL`
2. Define a DAO binding property as `DateTime`
3. Error: If you don't populate that DateTime property, it will be set to a default value, and throw a SQL exception (Property should be defined as `DateTime?`)

It would be nice if FastDAO could generate a NHibernate mapping and compare it against the defined mapping to see if there are any glaring errors such as a type mismatch when an InvalidCastException occurs.

Start by reproducing the DATETIME NULL and DateTime binding scenario described in the issue. Read the DAO binding behavior and the proposed NHibernate mapping comparison, then define what diagnostics should appear when an InvalidCastException results from a type mismatch. Done means the mismatch is detected and reported clearly.

Written by the indexing model from the issue text.