Added about_openstructs.rb

This group of tests shows that OpenStructs behave similarly to Hashes. I was hoping to copy this behavior for Structs, but it seems that Structs don't have the same Hash-like behavior that OpenStructs do.

Speaking of Structs, Struct functionality is not currently demonstrated in the Koans, so irosenb and I are going to add a file / a group of tests to demonstrate how to use them.

Thanks,
Stefan