move rimfar and mkdirp to devDependencies
Merge pull request #13 from danmilon/move-rimraf-mkdirp-to-devDependencies

move rimfar and mkdirp to devDependencies.
use streams2 when it exists. props @danmilon.
DRY code a bit.
DRY up tests.
zip: push data asap and use data descriptor. closes #14.
api: prefer create/append. groundwork for archive updating in future release!