Initial values

[drazik]

For now the initial values are given in the model prop. But the schema can contain initial values too. So we have a helper function like getInitialValue (not included in the lib as it's pretty specific to the entities from Symfony that we have to deal with in our projects) that takes the form and an object and tries to get the initial value or each field for that.

It means that the user have to merge initial values from the schema and from an entity. I think it would be better for the user to only give the data related to the entity, and let the Form component get initial value from the schema if it should be used (e.g. if the given model doesn't have a value for a given field, but the schema has one).