Add support for sql null types

[adam-frisby]

This pull request adds Null* types from the database/sql package to the Hood SQL Types and adds 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.

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Example:

type Test struct {
    Id       hood.Id
    Number   sql.NullInt64
}

// Connect and Start Transaction

a = Test{}
tx.Save(&a) //Number column is set to NULL in database (defaults to NULL)

b = Test{}
b.Number = sql.NullInt64{128, true} //Number column is set to 128 in database
tx.Save(&b)

c = Test{}
c.Number = sql.NullInt64{128, false} //Number column is set to NULL in database since valid bool is set to false
tx.Save(&c)

Retrieving the row which was inserted by tx.Save(&c) above will give you a sql.NullInt64{0, false} for Number

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Question: Since normal types (int, string, etc) can't represent a null value, should the notnull keyword be removed and assumed to be not null unless the column type is of type NullBool, NullInt64, etc?