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 
You are right @xphere, I missed that one. The example you gave can potentially enter in an endless loop, when `$other` is a `NullMoney`.

IMHO it is just about using a convention: we can say that comparing a `MoneyInterface` with a `NullMoney` would always return `-1` (minor than)

What do you think?