Composer support

[patrickdreyer]

We successfully integrated this great Yii functional testing solution into Behat, so that one can take full advantage of Behavior-Driven-Development, still having the functional tests running blazing fast without the overhead of running them through a Browser and web server.
As the whole Behat and Mink platform heavily depends on composer, gratefully, we'd like to port wunit for making use of composer as well.
However, for not forcing Yii users into composer, I suggest a mixed approach:

• Providing composer support through composer.json
• Continue to provide the required Symfony classes through the wunit repository
  Please let me know your thoughts.
  Cheers