[WIP][Workflow] Introducing the workflow component

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | not yet
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

TODO:

* [ ] Add tests
* [ ] Add PHP doc
* [ ] Add Symfony doc doc
* [ ] Add Symfony fullstack integration (Config, DIC, command to dump the state-machine into graphiz format)

This is basically a component that implements a state-machine.

Right now it's a simple implementation, but it can lead to a full implementation of [coloured Petri nets](http://en.wikipedia.org/wiki/Petri_net). A lot of things should be done before a merge.

So why another component?

This component take another approach that what you can find on [Packagist](https://packagist.org/search/?q=state%20machine).

Here, the statemachine is not tied to a specific object like with [Finite](https://github.com/yohang/Finite). It means that the component workflow is stateless and can be a symfony service.
Then the ObjectProperty can be anything, thanks to the PropertyPath Component.

(BTW, we are already using this component in production ;))

Some code:

```php
    use Symfony\Component\Workflow\Definition;
    use Symfony\Component\Workflow\Transition;
    use Symfony\Component\Workflow\Workflow;

    $fooDefinition = new Definition('Acme\\Bundle\\AcmeBundle\\Entity\\Foo');
    $fooDefinition->addStates(array(
        'new', 'pending', 'finished'
    ));

    //                                           name       from    to
    $fooDefinition->addTransition(new Transition('process', 'new', 'pending'));
    $fooDefinition->addTransition(new Transition('finish', 'pending', 'finished'));


    $dispatcher = new \Symfony\Component\EventDispatcher\EventDispatcher();
    $workflow = new Workflow('foo', $fooDefinition, $dispatcher);
    $workflow->setStateProperty('internalState');

    $foo = new Foo();
    $state = $workflow->getState($foor);
    echo $state; // 'new'

    if ($workflow->can($order, 'finish')) {
        // This is not possible to pass from "new" to "finished"
        $workflow->apply($order, 'finish');
    }

    $workflow->apply($order, 'process');

    $state = $workflow->getState($foor);
    echo $state; // 'pending'
```
``is_enabled`` is still far too generic. You can have many enabled things which have nothing to do with workflows (users can be enabled in Symfony for instance).

Thus, I don't see how ``is_enabled`` would give you the meaning of being allowed to follow this transition