Implement Pattern Matching

As pointed out here http://theerlangelist.com/article/macros_5 the following fails miserably too:

``` elixir
 defmodule Kwfuns.PatternMatchingTest do

  use ExUnit.Case
  use Kwfuns

  # For 0.1
  defkw fun(true, x), do: x 
  defkw fun(_, x), do: :error

  test "true case" do
    assert fun(true, 42) == 42
  end

  test "default case" do
    assert fun(42, 42) == :error
  end
end
```
