Implemented support for a different constructor arguments in a different context classes

From now on, there is no `parameters` section under the `suite`. Context parameters are passed
through `contexts` section in `behat.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:

  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.
Implemented context class resolvers.

Closes #420
Moved ContextClassGenerator into Context\ContextClass namespace