Feature/currency conversion

Implemented approximated amount preview on Product page. (Refs #132)
Currency converter uses https://openexchangerates.org/ API and refreshes his cache once per day (with first request without fresh data).
Implementation details are strongly based on https://github.com/evonove/django-money-rates but I adapted it to work with Prices and removed some code. Maybe we should propose our implementation as Pull Request (later).