Add 'zip' command to stapler

 - syntax is equal to sel/cat:
   from each pdf, the selected pages are merged in an interleaving manner
 - can be used to combine even and odd pages (close #3)
 - updated README.md and CONTRIBUTORS

The merge operation does NOT stop once one of the inputs is exhausted, but continues with the remaining input ([done in roundrobinLIST](https://github.com/slizzered/stapler/compare/hellerbarde:master...slizzered:feature-zip?expand=1#diff-1d3c64e3c3d420175ec4da112e680e5dR14)). If the user does not want the whole file, there is the possibility to use ranges.

More than 2 input files are supported, the algorithm will then interleave all these inputs.