Feature/Cancel transaction

### What?
Inspired in the cancelTransaction feature of Realm I've introduced it too in SugarRecord

### How?
If you are in a writing operation and you want to cancel the current changes you can do it by calling `cancelWriting()` in the SugarRecord context. What that actually does is:

- In CoreData it calls `rollBack()` in the NSManagedObjectContext to revert the existing changes.
- In Realm it calls `cancelWriteTransaction()` in the existing Realm

### Implications
- Added methods to the SugarRecord contexts
- Added methods to the SugarRecord extensions of Objects
- Added tests for that feature
- Updated the README.md to add a short description about that feature.