Added encryption to Anonymizer audit trail

Added a new :encryption option to the Anonymizer class, where a Digest
class can be given (e.g. Digest::SHA256).

This leads to the encryption of all 'original' DICOM values stored in
the audit trail file, and means you can store a record which maintains
a relationship between original DICOM values and their anonymous
replacement values, without actually storing any patient sensitive data!