Add countries.json to examples data dir

The examples cannot use files located in the test dir, or they won't work when hosted as GitHub pages.
Make vector-layer example ignore XHR errors

This is a workaround for our check-example.js PhantomJS script to work with this example. Without this check-example.js fails because xhr.status is 0. I tried to give the explicit file:/// scheme as indicated in https://groups.google.com/d/msg/phantomjs/wFGme0pE-Tk/WUjU5s-27NwJ, but that didn't work for me.
Merge pull request #381 from elemoine/countriesjson

The vector-layer example does not work when hosted