[WIP] Sugar record class

Hi Pedro,

I'm making my hands dirty touching the SugarRecord.swift class. I saw this morning the amazing work you did with the documentation and splitting the classes, so I couldn't resist to open my first PR :)

I will comment about this later in the email I have pending to reply, but essentially in this PR I wanted to get familiar with the code so I did a few tweaks meanwhile I was reading. I don't want to merge this PR until I have the test ready for testing the class methods of SugarRecord.swift to ensure I haven't broke anything.

So in this PR the changes so far are:

- Moved the files to the right path to match with the group on xcode
- I changed the KVO strings from var to let. I think the name of the constants doesn't follow the style guide.
- I saw all the class methods where returning tuples, so I removed it.
- More tweaks I comment inline


What do you proposes?, You can pass not only the queue but if you want that queue to be executed with a given priority