Wercker intergration

Hi,

it seems you have pretty good test pattern for this module, so I thought it would be good to show the current build status of the project, for example using [Wercker](http://wercker.com/).

Here are brief instructions on how to do it in case you're interested in integrating this repo with the service:

* [Register an account](https://app.wercker.com/users/new) to Wercker (it's possible to register with Github account).
* Create a new Wercker Application.
* Complete the wizard steps. All default settings seemed to work out of the box.
* Add _wercker.yml_ file in the root of the repository (in this pull request).
* After first build has been done, it's possible to add a badge to _Readme.md_ file. Markdown code can be found from the badge dropdown menu on the "Builds" overview page.

[Getting started](http://devcenter.wercker.com/articles/gettingstarted/web.html) guide might became helpful, too.

[See my fork](https://github.com/tuunanen/object-merge) of this repo in case you're interested in seeing how the outcome would look like.