[WIP] Add Subscription Bundle

This goal of this PR is to add subscription features to Sylius, to support recurring orders, club-memberships, etc.

![sylius_subscriptions1](https://f.cloud.github.com/assets/624921/1292758/12bd9388-3059-11e3-995b-dab26e900542.JPG)

![sylius_subscriptions2](https://f.cloud.github.com/assets/624921/1292759/1973b77a-3059-11e3-9e16-6389ad1fc227.JPG)

![sylius_subscriptions3](https://f.cloud.github.com/assets/624921/1292760/1bd1ccaa-3059-11e3-978d-2515b0befa5b.JPG)

The model revolves around subscriptions and subscription items, which on the application level can be linked specific products/variants. The scheduling mechanism is decoupled so that complex scheduling scenarios can be supported. A simple interval-based scheduler is included. The bundle also comes with a command and a processor harness to process subscriptions that are due. Actual processing logic is delegated to the application layer.

I also added a new section to the customer panel to allow customers to manage their subscriptions.

Feedback pleez!
Thanks! :)


Todos:
- [ ] Make Interval Units a dropdown and configurable
- [ ] Add twig extension to format interval unit + frequency, i.e. "Every 10 days / Every Month"
- [ ] Add Behat Features
- [ ] Add ability to create new subscriptions (from cart / during checkout)
- [ ] Add ability to add items to existing subscription (how to select product variant?)
- [ ] Add admin capabilities
- [ ] Add log / retry / audit capabilities (optional, could be separate bundle)

This is in the My Account area, which is protected by the access control in security.yml. So that scenario should not ever happen unless a user disables that check.

Do you think its worth it to add the getUser() === null check anyway? I haven't checked but I'd assume most of the my account area relies on this being secured from the user in the security.yml.