Add support for sql null types

This pull request adds Null* types from the database/sql package to the Hood SQL Types and adds the support for the types in SetModelValue. I've tested the changes with both mysql and postgresql. Everything looks to be working as expected. This will allow hood to handle values which may be null.

Questions: Since normal types can't be null in go, should the notnull keyword be removed and assumed unless the column type is of type NullBool, NullInt64, etc?