Add build status tag to README.
Add build status tag to README.

phatblat/CoreDataStack,bignerdranch/CoreDataStack,bignerdranch/CoreDataStack,phatblat/CoreDataStack,gaurav1981/CoreDataStack,Sully73/CoreDataStack,binhho/CoreDataStack,gaurav1981/CoreDataStack,bignerdranch/CoreDataStack,Sully73/CoreDataStack,binhho/CoreDataStack,sugar2010/CoreDataStack,sugar2010/CoreDataStack