"At common law a confessional statement made out of court by an accused person may not be admitted in evidence against him upon his trial for the crime to which it relates unless it is shown to have been voluntarily made. This means substantially that it has been made in the exercise of his free choice. If he speaks because he is overborne, his confessional statement cannot be received in evidence and it does not matter by what means he has been overborne. If his statement is the result of duress, intimidation, persistent importunity, or sustained or undue insistence or pressure, it cannot be voluntary. But it is also a definite rule of the common law that a confessional statement cannot be voluntary if it is preceded by an inducement held out by a person in authority and the inducement has not been removed before the statement is made..."