Fixes #148 #143 #141 and add tests using karma and mocha

- Partials are now relative to their parent
- I've also removed the need to use "_" as a path separator. It really looked like a hack. I much prefer being able to use `{{> partials/_footer }}` (if my partial file `_footer.hbs` is within a `partials/` subdirectory)
- I've cleaned the mess with requirejs so that it's much easier to include this plugin to any project without having to worry about `json2`, `Handlebars.js`, ...
- I've added tests with karma and mocha, you can run them with `npm test`
- I've updated the README to reflect all those changes