Up tool versions, fix warnings on rustler and add basic test

.tool-versions

@@ -1,2 +1,2 @@
-erlang 20.0
-elixir 1.9.0-otp-20
+erlang 21.0
+elixir 1.9.0-otp-21

native/xler_native/src/lib.rs

@@ -1,6 +1,6 @@
 #[macro_use] extern crate rustler;
-#[macro_use] extern crate rustler_codegen;
-#[macro_use] extern crate lazy_static;
+extern crate rustler_codegen;
+extern crate lazy_static;
 extern crate calamine;
 
 use rustler::{Env, NifResult, Encoder, Term};

test/xler_test.exs

@@ -1,8 +1,39 @@
 defmodule XlerTest do
   use ExUnit.Case
-  doctest Xler
+  # doctest Xler
 
-  test "greets the world" do
-    assert Xler.hello() == :world
+  def file(), do: File.cwd!() <> "/test/data/sample.xlsx"
+
+  test "can read worksheets from xlsx file" do
+    assert Xler.worksheets(file()) == {:ok, ["Sheet1"]}
+  end
+
+  test "gets a list of data returned via the worksheet name" do
+    {:ok, data} = Xler.parse(file(), "Sheet1")
+
+    assert length(data) == 701
+  end
+
+  test "returns the correct first row" do
+    {:ok, data} = Xler.parse(file(), "Sheet1")
+
+    assert data |> List.first() == [
+             "Segment",
+             "Country",
+             "Product",
+             "Discount Band",
+             "Units Sold",
+             "Manufacturing Price",
+             "Sale Price",
+             "Gross Sales",
+             "Discounts",
+             " Sales",
+             "COGS",
+             "Profit",
+             "Date",
+             "Month Number",
+             "Month Name",
+             "Year"
+           ]
   end
 end