method get() & count() were added to file_storage and db_storage

[santiagobedoa]

DBStorage and FileStorage updated.

Two new methods were added to both files:

1. A method to retrieve one object:

   • Prototype: def get(self, cls, id)
   • cls: class
   • id: string representing the object ID
   • RETURN: object based on the class and its ID, or None if not found.

2. A method to count the number of objects in storage:

   • Prototype: def count(self, cls=None)
   • cls: class (optional)
   • RETURN: number of objects in storage matching the given class. If no class is passed, returns the count of all objects in storage.

New tests were added for these 2 methods on each storage engine.