Add node webkit detection

should close #51 

node webkit add a `process` global object which contains a lot of information.
`process.versions['node-webkit']` contains node webkit version and can also be tested if someday another tool expose a `process` variable.

see:
* https://gist.github.com/thcreate/5771471
* https://github.com/bestiejs/platform.js/issues/18
* http://stackoverflow.com/questions/13726877/target-node-webkit-browser-using-jquery