Remove koans directory from source control.

Patches were submitted against the koans directory rather than the
src directory. This lead to potential problems when we regenerate the
koans directory from scratch, leading to the very real possibility
that changes could be lost.

Please make all changes to the src directory and use "rake gen" (or
"rake regen") to generate the koans directory as needed.

koans/about_to_str.rb

@@ -1,54 +0,0 @@
-require File.expand_path(File.dirname(__FILE__) + '/edgecase')
-
-class AboutToStr < EdgeCase::Koan
-
-  class CanNotBeTreatedAsString
-    def to_s
-      "non-string-like"
-    end
-  end
-
-  def test_to_s_returns_a_string_representation
-    not_like_a_string = CanNotBeTreatedAsString.new
-    assert_equal __, not_like_a_string.to_s
-  end
-
-  def test_normally_objects_cannot_be_used_where_strings_are_expected
-    assert_raise(___) do
-      File.exist?(CanNotBeTreatedAsString.new)
-    end
-  end
-
-  # ------------------------------------------------------------------
-
-  class CanBeTreatedAsString
-    def to_s
-      "string-like"
-    end
-
-    def to_str
-      to_s
-    end
-  end
-
-  def test_to_str_also_returns_a_string_representation
-    like_a_string = CanBeTreatedAsString.new
-    assert_equal __, like_a_string.to_str
-  end
-
-  def test_to_str_allows_objects_to_be_treated_as_strings
-    assert_equal __, File.exist?(CanBeTreatedAsString.new)
-  end
-
-  # ------------------------------------------------------------------
-
-  def acts_like_a_string?(string)
-    string = string.to_str if string.respond_to?(:to_str)
-    string.is_a?(String)
-  end
-
-  def test_user_defined_code_can_check_for_to_str
-    assert_equal __, acts_like_a_string?(CanNotBeTreatedAsString.new)
-    assert_equal __,  acts_like_a_string?(CanBeTreatedAsString.new)
-  end
-end