[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)

I'm not too familiar with translations, I could use some input on how best to do this if anybody has dealt with similar translations before...

For example, the commas are not valid in all countries, and the pluralization has to be accounted for as well. For now I'm going to have to punt on this since my solution would probably not be the best (just haven't dealt with this kind of stuff too much).