Move sigils to later in the lessons

While doing this, I also discovered that
there is also a reference to lists in numbers, but
that might a bit easier for someone to grasp, given
the hint that the koan gives, and the output they
see when they run it.

lib/koans/04_atoms.ex

@@ -0,0 +1,22 @@
+defmodule Atoms do
+  use Koans
+
+  @intro "Atoms"
+
+  koan "Atoms are constants where their name is their own value" do
+    adam = :human
+    assert adam == ___
+  end
+
+  koan "It is surprising to find out that booleans are atoms" do
+    assert is_atom(true) == ___
+    assert is_boolean(false) == ___
+    assert :true == ___
+    assert :false == ___
+  end
+
+  koan "Like booleans, the nil value is also an atom" do
+    assert is_atom(nil) == ___
+    assert :nil == ___
+  end
+end