Encapsulate the 'class cart' to `Class`

* I added `Class` service which acceses to `api/classes/:classNo`,
  using `$resource` service.
* I added `ClassCart` service which manipulates the 'class cart' in
  'create' section, of which the features were in `CreateCtrl`
  previously.

    * This service returns a class function so that 'class cart'
      objects can be created.
    * The constructor of this class accepts two parameters.

        * cart – Optional. The 'class cart' to be initialized with.
        * isStoredCart – Optional. `true` if `cart` is a 'class cart'
          which is stored in `storedContext`, or `false` if `cart` is
          a 'class cart' which is to be exposed to the scope of
          `CreateCtrl` controller.

    * The 'class cart' to be exposed to scope of controller can be
      accessed with `scopeCart` property.
    * The 'class cart' to be stored to cookie can be accessed with
      `storedCart` property.
    * A 'class' object can be added to the 'class cart' by calling
      `addClass` method.
    * A 'class' object can be deleted from the 'class cart' by calling
      `deleteClass` method with `class_no` of the 'class' object as
      the parameter.
    * A 'major' in a 'course' can be deleted from the 'class cart' by
      calling `deleteMajorInCourse` method with `code` of the 'major'
      object as the first parameter, and `course_no` of the 'course'
      as the second one.
    * A 'course' can be deleted from the 'class cart' by calling
      `deleteCourse` method with `course_no` of the 'course' as the
      parameter.

* I modified `MajorInfo` service.

    * Now this service acceses to `api/majors`, the url of which has
      no `/` at the head.
    * I added a function named `getNameByCode`, which returns a
      promise that is resolved with the name of 'major' of which the
      code is given parameter.

* I modified `config` constant.

    * I added a property named `isPlacementRequired`, which is a
      function that returns true if the 'class'es of 'course' of given
      'class' object should be placed by default, or false if not.

* I modified `CreateCtrl` controller.

    * I modified the dependency of this controller not to depend on
      `$q` and `Utils`.
    * I modified `deleteClass` function and added
      `deleteMajorInCourse` function and `deleteCourse` function to
      use the methods of `ClassCart` instance.