Added model population.

I still need to make changes on this, so don't close the PR after merging.
I have to change a few things and document some others, but it's usable, so you can continue developing other stuff.
Tests are failing in 64 bits, so I have to check what's happening there.
You can find usage examples in SCVModelTest, but basically, after defining a class, you can populate it with:
```objective-c
NSDictionary *dictionary; // a dictionary with the model data
NSError *error;
MyModelClass *modelObject = [MyModelClass populatedObjectWithObject:dictionary options:nil error:&error];
```