Better ends-with implementation, fixes #20

The current XPath expression does not correctly match trailing attribute values. For example this one:

```xml
<pluralRules locales="mo ro">
```

The result is that it (quietly) generates plural functions with empty body for `ro`. Not in scope here - but would it be possible to either fail or generate a function that fails (throws) for unsupported locales?

Based on [this post on stackoverflow](http://stackoverflow.com/a/22437888).