I18n: use system file separator

On Windows machines, `"/"` causes problems with namespaced resource bundles, turning a bundle like:
````
com.foo.bar.web.resource.MessageResource
`````
into
```
c:\myProject\src\resources\com/foo/bar/web/resource/MessageResource
````
So we end up with a `basename` on line 137 of `com/foo/bar/web/resource/MessageResource`, which blows up on line 145.

There's a pretty easy non-code workaround - in your pom.xml file, just replace this:
```
<bundle>com.foo.bar.web.resource.MessageResource</bundle>
```
with this:
```
<bundle>com${file.separator}foo${file.separator}bar${file.separator}web${file.separator}resource${file.separator}MessageResource</bundle>
```
But it seems better to handle it internally.