Extensions to the NSDecimalNumberHandler class for more Swift-like decimal number operations when using a custom decimal number behaviour

DecimalNumberHandlerExtensions.swift

extension NSDecimalNumberHandler {
    public func calculate(closure: () -> NSDecimalNumber) -> NSDecimalNumber {
        let defaultBehavior = NSDecimalNumber.defaultBehavior()
        NSDecimalNumber.setDefaultBehavior(self)
        let result = closure()
        NSDecimalNumber.setDefaultBehavior(defaultBehavior)
        return result
    }
}