Add option to put code in <head>

There are cases where putting analytics code in `<head>` is required, for example to verify a site with webmaster tools.

This PR adds a conf option to put code in `<head>` instead of `<body>`:

`config.middleware.use("Rack::GoogleAnalytics", :web_property_id => "UA-XXXXXXXX-1", :async => true, :position => :head)`

Code will be inserted in `<body>` if no `:position` option in provided.