Provide a CanonicalNumber property to expose the normalized standard book number

[Daniel Cazzulino]

Since we perform automatic "upgrading" of 10 digit ISBNs to 13 digit ones, in addition to
replacing the X at the end with 10, removing whitespace and dashes, and verifying checksums,
exposing a normalized form of the ISBN might be quite handy for ensuring consistent persistence
of book identifiers regardless of user input.