implement class dependency resolver to output Stubs.h which has no error in XCode.

I implement  IB::DependencyResolver class that work on IB::Parser#find_all.
Then, we can use Stubs.h without sort interfaces by my hand.

Before(in Stubs.h): 
```Objective-C
@interface SubSubClass: SubClass     // <- not defined SubClass and SubClass Interface, yet, then Error
@end

@interface SubClass: SuperClass        // <- not defined SuperClass Interface, yet
@end

@interface SuperClass: UIViewController
@end
```

After(in Stubs.h): resolving class dependency
```Objective-C
@interface SuperClass: UIViewController    // <- first define SuperClass Interface
@end

@interface SubClass: SuperClass               // <- already defined SuperClass Interface
@end

@interface SubSubClass: SubClass
@end
```
This dependency resolving algorithm is topological sort which in ruby's standard library.


And, other changes is following:

* move spec fixtures spec/fixtures -> spec/fixtures/common
* add fixtures for dependency test to 'spec/fixtures/depenendecy_test'
* add constraints to target only the files of under app/* directory
  * I think that other gems library files don't need on Stubs.h.
* and fix some test cases to support above changes.