Transparent NullMoney operations

Operations relative to other MoneyInterfaces objects are now
transparent (they do not return $this, but the comprared Money object).
This avoids that issuing such operations between Money and NullMoney
would return NullMoney instead of Money objects.

Setters were also deleted from MoneyInterface, since objects
implementing this interface will be value objects.

See issue #292 
Maybe we've been focusing in the wrong side of the problem.
If I recall correctly, `NullMoney` exists for cases when you have no `Currency` defined.
Maybe we can assume that, at zero amount, `Currency` is irrelevant.
This solution would be bad for, say, temperature conversion, but not for currencies.