Use on-the-fly zipping & streaming

As i had the same problem with zip-files of big directories i searched the net and found an interesting solution: http://stackoverflow.com/questions/4357073

I changed the code so that zip-files are genearted on-the-fly as a stream using the unix "zip" shell command. so this solution adds a new dependency, but as i suppose that a lot of people have "large" photo albums.....  

A problem could be that the filenames of ALL files that have to be added to the zip-file are listed as arguments on the command line and i'm not sure how long the command line can be? i have shortened the arguments by using only the relative path to the files. this has the advantage that also the created zip-archive has only the relative path of the files. 

This commit fixes #84.