[WIP] Sylius payum integration

The PR attempt to integration [payum](https://github.com/Payum) into sylius e-commerce.  Currently it contains two examples. First is offsite payment (paypal express checkout was used) and second credit card payment (stripe via omnipay bridge was used). 

#### Integration

I briefly describe integration steps required. Check [Files Changes](https://github.com/Sylius/Sylius/pull/275/files) for more details.

1. Add payum's lib to composer.json and install them
2. Update `PaymentMethods`. Remove all exist and add Stripe and Payal EC.
3. Add `PurchaseStep` to checkout scenario.
4. Configure payum

#### Order <-> Payment details relation problem 

We should definitely discuss the order <-> payment details relation problem. 

**Clarification:**

Payum  does not provide unified payment model in any way. All of them payment specific, do not share any interfaces or parent classes. For paypal rest situation even better: we reuse their official model that comes with `paypal/rest-api-sdk`. The bridge for jms plugings works with its `PaymentInstruction` model. Omnipay does not provide any model yet.

**How I solve it in customers projects**:

We have `Order` to `PaymentDetails` realtion (Order is owning side). Let's say one-to-one. `PaymentDetails` model implements class table inheritance pattern. There should be a way to dynamiclly add new classes to the map. I think it is possible with metadata doctrine event. I used this strategy in several projects: works fine but I doubt it is the best solution for the e-comerrace platform. 

**Solution:**

I added details property to `Paymnet` model (and its iterface). The details contains simple array. This way we do not need any extra models and can reuse current code. Payum native and omnipay's gateways have to work fine with this solution where jms plugins not. 

#### Moving payum related code to a bundle

What about moving all payum related stuff to payment bundle? or even better create one `SyliusPayumBundle`. It will contains all integration details.

**Solution:**

Moved.

#### Credit card processing questions?

Where to ask for credit card (if it is required)? It could be asked on payment method selection method (after user choose one) or inside `CaptureOrderWithXXXAction`.  

_**Note**: Since it is proof of concept everybody are welcome to discuss\criticize\ask about this PR. I am looking forward to get your feedback._

**Solution:**

It is asked on the separate page only if payment require it. 

#### TODO (maybe not in this PR (: )

* Fallback payments
* Simplify storages configurations
* Offline payments
* Allow set custom status for dummy payments.
* Design for credit card page.