[WIP] New stack structure for SugarRecord

The first big refactor in SugarRecord. Almost all the files have been removed and the Library is going to be redefined to use stacks and have communication interfaces between the main components of the library (**Persisted Objects**, **Sugar Record**, **Stack**).
User will be able then to initialize SugarRecord with the required stack and developers are going to be able to define their own stack and contribute to the library.
**Moreover it's going to have support to REALM! yeah!**

### Tasks
- [ ] Add tests to test the implementation and ensure every component is working
- [x] Define default CoreData stack
- [x] Review closures retaining
- [x] Explain an easy way to import the library into your project
- [ ] Comment the code and generate documentation with jazzy https://github.com/realm/jazzy
- [ ] Fill new README 
- [ ] Create playground tutorial

#### Don't forget

- Begin/End editing in database models for edition
- Difference between create and insert
![image](http://media.giphy.com/media/CF85cHN0DdoVa/giphy.gif)