This library allows reading and writing gzip-compressed JSON catalog files,
which can be used to store GPG, PKCS-7 and SHA-256 checksums for each file.  It
provides equivalent functionality to the catalog files supported in Microsoft
Windows.  Unlike Microsoft catalog files which are a signed manifest of hashes,
a Jcat file is a manifest of signatures.  This means it's possible (and
positively encouraged) to modify the .jcat file to add new signatures or replace
existing ones.
