[WIP] My account section : orders and invoices

Hi guys !

The goal of this PR is to create the second part of the account section which is about tracking orders and generating invoices. 

This PR refers to the following discussion : #88
A RFC describes the features and the implementation here : https://github.com/Sylius/Sylius/wiki/%5BRFC%5D-My-account-section

First I'd like to discuss about it (see pending questions). Once all pending questions will be resolved I'll write Behat scenarios and start to think about an implementation.

### RFC :

Allows the user to list all the orders he placed with the following information :
<table>
<tr>
 <th>Date</th>
 <th>Order number</th>
 <th>Amount</th>
 <th>Status</th>
 <th>Invoice</th>
 <th></th>
</tr>
<tr>
 <td>May 5, 2013</td>
 <td>30510110607C</td>
 <td>173,67 €</td>
 <td>Sent on May 8, 2013<br/>Tracking number : <a href="#">8V69692580122</a></td>
 <td><a href="#">PDF export of the invoice</a></td>
 <td><a href="#">Details</a></td>
</tr>
<tr>
 <td>June 22, 2012</td>
 <td>01228101495C</td>
 <td> 29,84 € </td>
 <td>Canceled by the customer</td>
 <td>-</td>
 <td>-</td>
</tr>
</table>

The Details link opens a popin with the details of the order:
* date, shipment mode, payment mode, order number, status, tracking number
* the list of products with thumbnail, name, unit price, amount
* shipping, amount total, a link to a PDF export of the invoice

### Pending questions : 
 - [x] when is an invoice available ? (when the user has payed, when his order has been validated in the back office ?????)
 - [x] which library would you use to generate a PDF export ? (I know KnpSnappyBundle which is a wrapper for wkhtmltopdf => quite simple and works well)
 - [x] split orders
 - [x] generating the invoices
 - [x] storing the invoices

### Answers to questions :
 - the invoice is available once the order is placed for the moment
 - KnpSnappyBundle will be used to generate invoice PDF
 - a better split order management could be done in another PR
 - invoices will not be stored as they will be rendered on demand for the moment