Adding sitemaps

Up for discussion. Please don't merge. I need to test a couple of things with webmaster tools before going live with these.

This PR's adds
- A nodejs script to create/update the sitemap files
- The sitemaps created by the above scripts
- Adding the sitemap to the robots.txt file

The following sitemaps are created right now by the script (and added to this PR):
- base.xml -> contains main page in 5 languages
- topics.xml -> contains all current topics (12) in all 5 languages
- layers.xml -> contains all layers of all topics in all 5 languages (results in 5494 entries, and around 1MB in size, gziped < 60KB)

The sitemaps above are referenced in the main sitemap.xml

Note that search engines recommend (uncompressed) files < 10MB and less than 50k entries per file.

To re-create the Sitemaps using current state @ api3.geo.admin.ch:
```
make updatesitemaps
```

This is a manual process, much like updateol.