Add default HttpKernel Bridge, Symfony Bootstrap and StackPHP.

 * Renamed Bridges/Symfony to Bridges/HttpKernel.
 * Move responsibility of bootstrapping back into the application.
 * Provided a default "Symfony" bootstrap.
 * Add StackPHP's Stack\Builder to decorate our HttpKernelInterface implementing app.
Update README.
Merge pull request #5 from trq/feature/http-kernel-bridge

HttpKernel bridge bootstrapped from the application itself.