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.go

@@ -0,0 +1,26 @@
+// Position support for go-toml
+//
+// BSD Licensed
+// Copyright 2014 eric.t.anderton@gmail.com
+
+package toml
+
+import (
+	"fmt"
+)
+
+// position within a TOML document
+type Position struct {
+	Line int // line within the document
+	Col  int // column within the line
+}
+
+// String representation of the position.
+// Displays 1-indexed line and column numbers.
+func (p *Position) String() string {
+	return fmt.Sprintf("(%d, %d)", p.Line, p.Col)
+}
+
+func (p *Position) Invalid() bool {
+	return p.Line <= 0 || p.Col <= 0
+}