"this." defaults to ".when" object

Make sure the default "this." becomes the object you called ".when" on. Now you can write:

person.when(["firstName", "lastName"], function (firstName, lastName) {
  this.fullName = firstName + " " + lastName;
});
Updated for lint
Update README.md
Merge pull request #4 from mathiasrw/gh-pages

Default "this" improved for ".when"