This causes a JAR to be fetched over a local HTTP server in the
e2e test harness instead of the external Jenkins mirror. Also causes the
JAR input to be randomized so we can re-use the createSignedJar method
in multiple tests.

Fixes: #375

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

* Improve code sharing in pluggable type support

This patch removes some of the duplicate logic from specific type implementations and moves it into the base design which hopefully makes writing pluggable types a bit easier.

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

* 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>