First unit test & refactored Swiffer.js to be testable

* Added mocha as test runner and expect.js as assertion library
* Added sinon as test helper library
* Added `npm test` command to run unit tests
* Added first unit test to test `swiffer.clean` method
* Updated package.json to include a project version
* Updated package.json to point at new binary script bin/swiffer
Merge pull request #1 from sethmcl/master

First unit test & refactored Swiffer.js to be testable