Feature changes

• Added <currency> argument to /balance <player> [<currency>]
• Added <currency> argument to /pay <player> [<currency>]
• Added new conversion calculation subcommand /currency convert <fromCurrency> <toCurrency> <amount>
• Added new messages and placeholders to give better support for multi-currency usage

Code changes

• Improved feature tests
• Added CurrencyUtil for easier reusage