More .Net Zip Library/Utility updates

[Update: this code has been moved to a codeplex project:   <www.codeplex.com/DotNetZip> .  Go there for the latest source for ths .NET Zip/compression library. It does ZIP, ZIP64, SFX, passwords, unicode, and streams. All zips are fully interoperable. ]  

I updated the Zip library again, this time to support the adding of fully qualified files or directories.

A couple people commented that the library did not handle fully-qualified pathnames properly.  Hmmm, I guess I need to embrace Test Driven Development a little more tightly eh?  The zipfile that got produced would be un-readable by Windows. 

I've changed the zip library to correct that.