fix examples; use async.forEachSeries

The examples were using async.forEach as a
way to asynchronously iterate over the files
to add them in the archives. This is problematic
because the library expects one file packaging
to be over before calling .addFile again.

Using forEachSeries fixes this, because it
iterates in series, not in parallel.
make examples runnable by including sample files
Merge pull request #11 from danmilon/fix-examples

Fix examples & make them runnable.