Issue #203

CurrencyBundle has two hard dependencies.

* LanguageBundle - Locale->getIso() is required for printing a currency
* Twig - Required for the Twig Extension

Both dependencies should be moved into suggested section in composer, to start uncoupling some packages.

To uncouple LanguageBundle locale implementation from CurrencyBundle, some adapters could be implemented, with dummy adapter by default, and with possibility of Elcodi implementation adapter usage.

To uncouple Twig (change twig/twig to symfony/twig-bundle) just skip extension tests if class does not exist. To make tests work, just add into travis-ci configuration file 'composer require --no-update symfony/twig-bundle ~2.4`. Do the same in main package