Merge pull request #9 from davidepedone/master

fixed _unwrap function for value=0
Made `callback`s optional. So it's now possible to use a syncron syntax. The old syntax should also work well. Push : Bugfix for the value `0`
changed version in docs and format
added newer node versions to travis test
Fix a few typos noticed in the readme.
Merge pull request #10 from mjschranz/readmeTypos

Fix a few typos noticed in the readme.