Add fallback host when examples are accessed through file:-protocol.

When opening an example through the `file:`-protocol, e.g.
`file:///ol3/examples/full-screen.html` our `loader.js` would inject
a `<script>` tag with a `src` pointing to an illegal URL
`http://:9810/compile?mode=ADVANCED&id=full-screen`.

This changes the respective logic, so that in the case that we are accessed
through the `file:`-protocol, we fallback to using `localhost` as hostname
for the plovr-compiler.

The examples that do not use XHR (like e.g. `wms-capabilities.html`) now
work as expected when accessed through `file:`-protocol.