Add new type for JAR archives (#272)

* Add new type for JAR archives

This adds support for a new pluggable type that can extract signatures
from signed JAR files. Per the JAR spec, a special manifest file is
created with the digest hashes of all included content in the JAR file.
It is this special manifest file that is then signed, and included in a
file within the archive in PKCS7 format. The PKCS7 file also includes
the X509 certificate that can be used to verify the signed manifest file
inside of the JAR.

Signed-off-by: Bob Callaway <bob.callaway@gmail.com>