Create es-es.txt

Here's a Castilian Spanish (from Spain, rather than South America) translation of the Perch UI. I hope it's useful. I'm an avid Percher and my wife and I worked together on it so hopefully it's all correct. She's a native speaker but didn't know some of the techie terminology, so we did a bit of homework and asking around in places (for example 'slug' doesn't translate exactly!). Thanks [htmlboy](https://github.com/htmlboy) and [martuishere](https://github.com/martuishere)!