[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)

Actually the route names are a little different from default since this is in the my account section - sylius_account_subscription vs sylius_subscription (default).

Also this functionality redirects the user to the index (instead of the show page which is default), which I feel is more useful since the subscription area is pretty basic.

Either way, if it would make more sense I can change the naming scheme so this uses more default values but it looks like most of the my account sub-sections follow a similar pattern.