Autoboxing for scalar/struct attribute values

Implemented my idea from #61. Allows you to write code like this (without using number literals and other ugly stuff):
```
make.top.equalTo(42);
make.height.equalTo(20);
make.size.equalTo(CGSizeMake(50, 100));
make.edges.equalTo(UIEdgeInsetsMake(10, 0, 10, 0));
```
Old code still works fine :)

Things I'm gonna do before this is ready for merging:

- [ ] Add tests
- [ ] Update/add docs
- [ ] Update examples
- [ ] Implement the same thing for `offset`? This would effectively ditch `centerOffset` and `pointOffset`.