Allow relative paths to be stored in package Zip

This pull request merges changes on the gklyne branch with master.  There are two functional changes:

1. The main change is logic to allow relative file paths to be used for zip package file names if a "basedir=" argument is supplied when creating the package.  Without this argument, the original behaviour of saving filenames only (without paths) is maintained.,

2. A message sent to stdout when initialising a package is suppressed.  This message was interfering with the desired output from my command line tool (intended to facilitate use of the tool as part of a command pipeline). This is at dip.py line 1257.  If you really want this message to show, I suggest sending it to stderr.