Support for timestamped signed modules

[Sean Mullan]

Please review the following webrev which adds support for timestamped signed 
modules.

webrev: http://cr.openjdk.java.net/~mullan/jigsaw/webrevs/timestamp/webrev.00/

A new option (tsa) has been added to jpkg to specify the timestamping authority 
to use. I have also renamed some of the signing options to be more descriptive. 
Here is an example of creating a timestamped signed module :

jpkg --sign --alias duke --keystore keystore \
      --tsa https://timestamp.geotrust.com/tsa \
      -m lib/test.security jmod test.security

A lot of the changes involved restructuring so that jars and modules could share 
the same timestamping code.

Thanks,
Sean