Added factory inheritance

[tillsc]

Added support for hash key :super to add inheritance of factories.

Use the key :super with value nil or with a fixture name to inherit from the fixture of the superclass.

E.g. (Duck < Animal)

Animal.fix {{
  :alive => true 
}}

Duck.fix {{
  :super => nil,
  :legs => 2
}}

If you've got a named fixture you can use the name instead of nil (which defaults to the name of the current factory)

Animal.fix(:dead) {{
  :alive => false
}}

Duck.fix(:fried) {{
  :super => :dead
}}

In the last case :super => nil won't work due to the fact that there is no Animal fixture with name `:fired