Added integration with Aelia Currency Switcher

The integration will allow to sell bookable products in multiple currencies using our [Currency Switcher](http://aelia.co).

Thanks to our Currency Switcher, product prices can be set manually as well (i.e. admins can set a product price to 100 USD, 100 EUR and 100 GBP, instead of relying on conversions). The integration is based on a loose dependency, i.e. the code will work whether our Currency Switcher is installed or not.

**Notes**

* We added code documentation to highlight the reason of the changes. We follow PhpDoc standard.
* You might want to review the code styling, as we use our own internal style, which might not match yours. :)