Add +[NSManagedObject find:] collection operator

- Makes best effort to perform more efficient search for a single record using
NSFetchRequest.fetchLimit

- Does not create a record when no matches are found
Expose -[NSManagedObject where:limit:]
Restore fetchWithPredicate:inContext:

- use default fetch limit of nil (unbounded)
Make limit tests more clear
Resolve conflicts
Merge pull request #50 from kattrali/find-without-create

Add +[NSManagedObject find:] collection operator