Avoid SKYReference in relational queries

[chpapa]

Chat with some team member, people keep asking why we need to do this:

SKYRecord *currentUser = [SKYContainer defaultContainer].auth.currentUser;
SKYReference *nameRef = [SKYReference referenceWithRecord:currentUser];

NSPredicate *accountPredicate = [NSPredicate predicateWithFormat:@"account = %@", nameRef];

instead of simply this:

SKYRecord *currentUser = [SKYContainer defaultContainer].auth.currentUser;
NSPredicate *accountPredicate = [NSPredicate predicateWithFormat:@"account = %@", currentUser];

I'm not sure if NSPredicate allow user to directly put an object there or not. So it maybe more like a swift only things, I don't know :(

If we can detect and support all of these, would be great:

• SKYReference (for backward compatibility)
• anything extended skygear.Record
• a string, assume it is a ID

That will greatly enhance skygear iOS SDK usability.