Added about_symbol_array_literals.rb.

This branch includes a demonstration of symbol array literals using %i and %I in Ruby 2.0. The two koans in this file show the effects of creating arrays using regular literals and interpolated literals. This branch follows the convention of my previous branches for which I've sent pull requests: the test classes are small (they have one or only a few koans each), so it might be better to add them to previously existing koan files and enclose them in conditionals testing whether the user is using Ruby 2.0. If we were to do this with symbol array literals, the file to append the koans to would likely be about_symbols.rb.

Again, please let us know if anything should be changed or added.

Thanks,
Stefan