feat(Squarer): add module to return the square of a given number

The Squarer module simply provides a square function that returns the
square result of the number provided.

The application has also had a supervisor added to enable basic running
of the program.

lib/elixir_poolboy_example.ex

@@ -1,2 +1,14 @@
 defmodule ElixirPoolboyExample do
+  use Application
+
+  def start(_type, _args) do
+    children = []
+
+    options = [
+      strategy: :one_for_one,
+      name: ElixirPoolboyExample.Supervisor
+    ]
+
+    Supervisor.start_link(children, options)
+  end
 end

lib/elixir_poolboy_example/squarer.ex

@@ -0,0 +1,5 @@
+defmodule ElixirPoolboyExample.Squarer do
+  def square(x) do
+    x * x
+  end
+end

mix.exs

@@ -14,7 +14,10 @@ defmodule ElixirPoolboyExample.Mixfile do
   #
   # Type `mix help compile.app` for more information
   def application do
-    [applications: [:logger]]
+    [
+      mod: {ElixirPoolboyExample, []},
+      applications: [:logger]
+    ]
   end
 
   # Dependencies can be Hex packages: