Add comment support

This commit add the ability to read and generate comments on the zip
file and/or on each zip entry.
Fix #134.
update build dependencies

The old version of the saucelabs plugin may explain why I couldn't run the
saucelabs tests.
I'm also upgrading the other build dependencies.
move ZipObject and JSZip attributes

Now `date`, `dir` and `comment` are attributes of `ZipObject`.
See #137 for more details.
Improve backward compatibility for ZipObject

If the user changes the metadata after the creation of the ZipObject AND
uses `JSZip#file` to access directly to the ZipObject, he will get a
bug. With this patch, we now store the initial values of dir and date
(changing dir afterward is a bit dangerous but you can't guess what a
user would do).
If the date (or dir) has been updated via the new path, we use the new
value. Otherwise, we use the value stored at the old path. That way, we
don't need to check the old path against the initial value.
Merge pull request #137 from dduponchel/comments

Add comment support