Money\Money is allowed to have nullable values

Description of the problem:
If you have an entity, that could have a price (optional), and the price is represented as an instance of Money\Money, as Embedded Entity.
Then it will create a db schema with all fields (price_amout, price_currency) but these fields are nullable = FALSE by default, so the price is not possible to track as OPTIONAL property of an entity.

In order to solve that, I propose to allow nullables.
Merge pull request #1 from prolic/patch-1

Money\Money is allowed to have nullable values