[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! :)