Fixed logic used to retrieve the currency

* Replaced all get_option('woocommerce_currency') with get_woocommerce_currency(). The latter will return the currency that is active when an order is being paid. In most cases, it will match the base currency, and it will also work with multi-currency plugins such as the Currency Switcher (http://bit.ly/1rVC5S1ac).
Merge branch 'dev' of https://github.com/aelia-co/paypal-woocommerce into aelia-co-dev
Merge branch 'aelia-co-dev' into dev