Make the `save` method returns BOOL

[Nicolas Brassard]

Here: https://github.com/soffes/ssdatakit/blob/master/SSDataKit/SSManagedObject.m#L348

From Apple's doc:

• (BOOL)save:(NSError **)error
  returns YES if the save succeeds, otherwise NO.

That would be great to be able to get that value, while still using simpler code, i.e. [myObject save];

Thanks!

[Nicolas Brassard]

Thanks!