added setDefaultModelFromClass for test purpose

[nanashi]

Support for Xcode Unit Test.

Example: in test case file

+(void)setUp{
  [super setUp];
  [[CoreDataManager sharedManager] setDefaultModelFromClass:[self class]];
  [[CoreDataManager sharedManager] useInMemoryStore];
}

Inspired by http://www.cimgf.com/2012/05/15/unit-testing-with-core-data/ "Supporting Xcode’s Unit Test Support" Section