Triangle facts feature

Adds angles properties to Triangles class.

Attempts to reduce some of the duplication in calculate_angles by creating a law_of_cosines method and positions array.  I can see how the original three lines are more clear, but it was kind of an exercise in DRY?
avoid such long lines. Among other things, attributes or an array would be better than the hash here. I also see a fair amount of duplication with sides supplied in different orders. should perhaps get pulled out into a method?