adding option to provide an attribute column

[djarecka]

closes #135

Adding support for attribute column, so attributes and slots could be used in one schema, e.g.:

class	attribute	slot	range	desc
>class	attribute	slot	range	description
ClassA				class A
ClassA		        slot_a	string	slot a
ClassA		        slot_b	int	slot b
ClassA	attr_a		string	        attribute a
ClassA	attr_b		int	        attribute b

gives:

slots:
  slot_a:
    from_schema: https://identifiers.org/brain-bican/kb-model
  slot_b:
    from_schema: https://identifiers.org/brain-bican/kb-model
classes:
  ClassA:
    description: class A
    from_schema: https://identifiers.org/brain-bican/kb-model
    slots:
    - slot_a
    - slot_b
    slot_usage:
      slot_a:
        description: slot a
        range: string
      slot_b:
        description: slot b
        range: int
    attributes:
      attr_a:
        description: attribute a
        from_schema: https://identifiers.org/brain-bican/kb-model
        range: string
      attr_b:
        description: attribute b
        from_schema: https://identifiers.org/brain-bican/kb-model
        range: int

but not sure if this is the best way to do it...