Dont store the currency object on the money, store the currency code

I thought this PR could be more like the start of a conversation.  I haven't done tests or changed the readme, as I wasn't sure your reaction.  

Basically for my usecase I want to store these Money objects in a JSON database (mongo in this case) and I don't want to store the entire currency object.  Also I'm thinking that over time, it'd be better to move to someone's more standard/comprehensive take on currencies as they change more often than you might think.

But that's really a side point, I'm more trying to make them serialize smaller and support the case where you're reading from a datasource that has ISO currency codes, but not references to the object itself.  

BTW, this library is exactly what I was looking for and I super appreciate your work here.

cheers,
andy