Fix examples & make them runnable

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 `async.forEachSeries` fixes this, because it iterates in series, not in parallel.

The second commit includes two sample files, so the examples are runnable straight away.

See #10 also.