Recognize that `TempFile` is a file.

Since `wkhtmltoimage` [has problem when HTML is passed in stdin](http://code.google.com/p/wkhtmltopdf/issues/detail?id=534), one solution is to store HTML in a temporary file, and pass this file to `IMGKit.new`.

This commit ensures that IMGKit accepts not only instances of `File`, but also of Ruby's `TempFile` as a way to declare that the HTML is stored inside a file.