Declare each currency converter class in it's own file

All currency converter classes are located in same file, not in separate files as they should. Base currency converter class is named "kCurrencyRates" (starting with "k"), but it is not part of In-Portal distribution, and belongs to In-Commerce module, so "k" should be removed. Also this class is registered not in unit config, but in kApplication class directly.

At the end all classes from file "\in-commerce\units\currencies\currency_rates.php" should be moved to "\in-commerce\units\helpers" folder and named properly, because they are actually helpers.