Context arguments

Depends on #418 

From now on, there is no `parameters` section under the `suite`. Context parameters are passed
through `contexts` section in `behat.yml`:

```yml
  default:
    suites:
      customer:
        contexts: [ CustomerContext, PackageContext ]
    contexts:
      CustomerContext:
        - value1
        - value2
      PackageContext:
        - http://behat.org
```

Those parameters also could be a placeholders, automatically resolvable by custom context argument
resolvers. Context argument resolvers are extensions implementing `ArgumentResolver` interface that
could add new ways of integration with custom framework. For example, this:

```yml
  default:
    contexts:
      YourApp\CustomerBundle\Features\Context\CustomerContext:
        @app:customer.repository
```

in case of Symfony2Extension could load `customer.repository` service from application container
and pass it as a CustomerContext constructor argument. /cc @stof