use pointer when load ffm model and release model with Explicit method

[Imagle]

1. Release model(W) not in destructor method.
2. Not to use ffm_load_model, because this method return a object which is local variables, when it returned, it will call destructor(~ffm_model()), and W will be freed.
3. Add a new method(ffm_load_model_ptr) which return a pointer to the model.
4. Add a new class method of ffm_model--release, which is used to free model in explicit way.