Setters for Money value object

Although setters for individual fields should not be allowed
in value objects, it was necessary to implement setCurrency and
setAmount in order to make the class collaborate with a Symfony
form types. Internally, the Form component uses setters to "attach" form
data to entity classes, so public setters were needed

See https://github.com/elcodi/bamboo-admin/issues/3
Thanks @hacfi. Fixed :)