Google Yahoo ECB exchange rate services, CLI command, admin button for u...

...pdate all rates, admin page for select active provider

We created feature for updating Exchange Rates from various providers. This feature is covered with phpspec, but not with Behat.

What we need is code-review before we try to make this PR ready. This PR is not fresh and it need some changes before it can be used. Also it can be used as idea or partially.

So please be kind and review the code. This PR is created because we wanted to contribute to Sylius and learn more about your structure. We are not primary symfony developer so every critique is welcome.

This is work in progress PR. This PR is not updated with master branch, and it requires some refactoring to make it compatible.

Features:

- Drivers for exchange rate services (Google, Yahoo, ECB)
- Command for updating exchange rates
- Added button on exchange rates page in admin to update all exchange rates
- Added new page in admin to select active exchange rate provider

This PR should be synced with #1831 changes.