Specs:

* Triangle
  * initializer
    - given 3 lengths
    - validates triangle cannot have any negative length.
    - validates triangle cannot have any zero length.
  * get_a
    - returns the correct value for length a
  * get_b
    - returns the correct value for length b
  * get_c
    - returns the correct value for length c
  * get_perimeter
    - calculates perimeter of triangle

* RightTriangle
  * initializer
    - given 2 lengths (a and b)
    - hypotenuse (c) is correctly computed
  * get_area
    - calculates area of right triangle object