Multi-currency support: Money + Currency class improvements (#553)

* Money improvements

* Replace all old money usage

lib/money/arithmetic.rb

@@ -0,0 +1,62 @@
+module Money::Arithmetic
+    CoercedNumeric = Struct.new(:value)
+
+    def +(other)
+        if other.is_a?(Money)
+            self.class.new(amount + other.amount, currency)
+        else
+            value = other.is_a?(CoercedNumeric) ? other.value : other
+            self.class.new(amount + value, currency)
+        end
+    end
+
+    def -(other)
+        if other.is_a?(Money)
+            self.class.new(amount - other.amount, currency)
+        else
+            value = other.is_a?(CoercedNumeric) ? other.value : other
+            self.class.new(amount - value, currency)
+        end
+    end
+
+    def -@
+        self.class.new(-amount, currency)
+    end
+
+    def *(other)
+        raise TypeError, "Can't multiply Money by Money, use Numeric instead" if other.is_a?(self.class)
+        value = other.is_a?(CoercedNumeric) ? other.value : other
+        self.class.new(amount * value, currency)
+    end
+
+    def /(other)
+        if other.is_a?(self.class)
+            amount / other.amount
+        else
+            raise TypeError, "can't divide Numeric by Money" if other.is_a?(CoercedNumeric)
+            self.class.new(amount / other, currency)
+        end
+    end
+
+    def abs
+        self.class.new(amount.abs, currency)
+    end
+
+    def zero?
+        amount.zero?
+    end
+
+    def negative?
+        amount.negative?
+    end
+
+    def positive?
+        amount.positive?
+    end
+
+    # Override Ruby's coerce method so the order of operands doesn't matter
+    # Wrap in Coerced so we can distinguish between Money and other types
+    def coerce(other)
+        [ self, CoercedNumeric.new(other) ]
+    end
+end