Storage get count

[yam1er]

Update DBStorage and FileStorage, adding two new methods. All changes should be done in the branch storage_get_count:

A method to retrieve one object:

Prototype: def get(self, cls, id): cls: class id: string representing the object ID Returns the object based on the class and its ID, or None if not found A method to count the number of objects in storage:

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