IMPLEMENT support of custom scalar types (/!\ swift not supported)

This is my proposition tu resolve issue #162.

To specify a custom scalar type you have to add to your Entity attribute user info a new item with key "attributeValueScalarType" and your custom scalar type as value.

During generation, for int16_t, int32_t, int64_t scalar types, if "attributeValueScalarType" key exists, the custom scalar type is copied into generated file.

If this custom scalar type is not visible from the machine generated header file (i.e. : not present in your pch file), it is possible to define an additional header file name.

It is possible to add to your Entity user info a new item with key "additionalHeaderFileName" and your additional header file name.

During generation, if this key is present, a new import line is added to the generated machine header file.

Warning : Swift is not supported, I do not know (yet) Swift. Feel free to add this support.