Add fibonacci numbers exercise

fibonacci_numbers.lua

@@ -0,0 +1,30 @@
+-- Solution --------------------------------------------------------------------
+function nth_fibonacci_number(n)
+	-- Your implementation here
+end
+
+-- Tests -----------------------------------------------------------------------
+
+local luaunit = require("luaunit.luaunit")
+
+function test_3rd_fibonacci_number_is_2()
+	luaunit.assertEquals(nth_fibonacci_number(3), 2)
+end
+
+function test_5th_fibonacci_number_is_5()
+	luaunit.assertEquals(nth_fibonacci_number(5), 5)
+end
+
+function test_10th_fibonacci_number_is_55()
+	luaunit.assertEquals(nth_fibonacci_number(10), 55)
+end
+
+function test_20th_fibonacci_number_is_6765()
+	luaunit.assertEquals(nth_fibonacci_number(20), 6765)
+end
+
+function test_80th_fibonacci_number_is_23416728348467685()
+	luaunit.assertEquals(nth_fibonacci_number(80), 23416728348467685)
+end
+
+os.exit(luaunit.LuaUnit.run())