[WIP] Use Zipruby instead of Rubyzip

Hi There,

I'm working on using adding passbook integration, and my requirements require me use zipruby as opposed to rubyzip. Since zipruby is faster due it being implemented in C, I thought I would offer it up on a PR.

I'm looking for feedback to see if I can make this better, and to see if you would be interested in having this.

Thanks!
Not sure that we need buf