NOT READY FOR MERGE. `SQKCoreDataOperation` example with data synchronisation.

**This branch is not ready to be merged yet. Pull request created to proved a discussion forum for improvements.**

This is an attempt to demonstrate the correct usage for `SQKCoreDataOperation` subclasses and how they can be used to synchronise data from a remote resource.

In this example, data from the Github API is imported in to Core Data; specifically [commits](https://developer.github.com/v3/repos/comments/) from the SQKDataKit repo and their associated [users](https://developer.github.com/v3/users/) are imported.

Areas for improvement:
 * `CDOCommitImportOperation` is added as a dependency to `CDOUserImportOperation` so that the `CDOUserImportOperation` is executed when the `CDOCommitImportOperation` is finished. However the `CDOUserImportOperation` does not inspect the state of the finished `CDOCommitImportOperation` to check to see if any error occurred. How should errors propagate back up to the `CDODataSynchroniser` from operations if they are added as dependencies to each other and the `CDODataSynchroniser` does not set completion blocks on them?
 
If anyone can think of any more improvements this list can be extended.