Change go.tools import path to golang.org/x/tools

The import path of go.tools are changed from "code.google.com/p/go.*" to
"golang.org/x/*".

see:
https://groups.google.com/forum/#!topic/golang-announce/eD8dh3T9yyA
Merge pull request #13 from harukasan/fix_import_path

Change go.tools import path to golang.org/x/tools