Money coercion

Working with `Money` you have to be careful about the order when using math operators. It shouldn't matter.

Otherwise you get

```ruby
1 + Money.new(3)
# => TypeError: Money can't be coerced into Fixnum
```

This PR allows money to coerce `Numeric`. 

```ruby
1 + Money.new(3)
# => #<Money value:4.00>
4 * Money.new(3)
# => #<Money value:12.00>
```

@jduff  @mcgain