pdb/mobi: Convert fields to String, add Check

The type and creator fields of the PDB header should be "BOOK" and
"MOBI" for mobi documents. This patch converts those fields to String
types of length 4 and adds a check in MobiFile.parse(PdbFile), throwing
an InvalidHeaderException if they aren't equal to our constants.
inspect: Convert to PdbFile/MobiFile

Instead of extracting each header using the MarshalBuilder, Inspect
should use the new container types for parsing. Both to ensure wrap that
functionality up and to ensure that MobiFile is tested sufficiently
before it's real debut.