Added Position Support to TomlTree

TomlDocument provides an optional TOML processing path where position
informaiton is stored alongside a TomlTree.
* Added Position struct
* Revised TomlTree to contain position data
* Added tomlValue to bind positions to values
* Revised parser to emit position data
* Revised token to use new Position struct
* Added tests for new functionality
* Bugfixed table array duplicate key handling
* Applied gofmt to all code

position_test.go

@@ -0,0 +1,31 @@
+// Testing support for go-toml
+//
+// BSD Licensed
+// Copyright 2014 eric.t.anderton@gmail.com
+package toml
+
+import (
+	"testing"
+)
+
+func TestPositionString(t *testing.T) {
+	p := Position{123, 456}
+	expected := "(123, 456)"
+	value := p.String()
+
+	if value != expected {
+		t.Errorf("Expected %v, got %v instead", expected, value)
+	}
+}
+
+func TestInvalid(t *testing.T) {
+	for i, v := range []Position{
+		Position{0, 1234},
+		Position{1234, 0},
+		Position{0, 0},
+	} {
+		if !v.Invalid() {
+			t.Errorf("Position at %v is valid: %v", i, v)
+		}
+	}
+}