Add 3rd default param

Unpack.get/3 takes a default value to return when key not found in get function.

Ecto is back!

Thanks to @alexpeachey for a great solution for Ecto.Association.NotLoaded!

No ecto

Reverted to Elixir 1.6
Removed handling of %Ecto.Association.NotLoaded{}.
Easy enough for users of this hex package to catch any unwanted values returned from Unpack.get/2

Correctly handle maps and structs.

The first pass of Unpack handled structs and maps. This was refactored to utilize the Kernel.get_in/2 function. However, this function does not work for Structs. This release removes the Kernel function call and adds better tests.