A 2-years-in-the-making cleanup! Use Go 1.4 go/format!

Now that issue 5551 has finally been fixed, and Go 1.4 is out, all this
hacky and messy workaround code can be removed! Woohoo!
Also remove other unused imports and avoid dot imports.
For more details, see commit message of shurcooL/go@7ff9dcacbfedec7ab78.