Removed saveMainContex: nested, reference issue #15

It doesn't really fit with the responsibilities for the context manager,
and it does't really do much other than call save: anyway.

Current the `testMergePropagatesChangesWhenMergingPrivateContextIsSaved` unit tests doesn't pass so don't merge.

Addresses [this issue](https://github.com/3squared/SQKDataKit/issues/15).