Rss support for tags

closes #2260
- added routes for /tag/:slug/rss and /tag/:slug/rss/:page
- added support for tag in the rss controller
- added route tests for each extra case
- fixing a tiny typo in some test descriptions
Updating copyright range

Updating the copyright year to be a range. :-)
Otherwise the license wouldn't explicit cover the older versions you've made.
Serve default robots.txt

closes #2062
- Server robots.txt from theme if available
- Serve default robots.txt from /core/shared/ otherwise
- Added tests for default robots.txt
Merge pull request #2341 from stenehall/tag_based_rss_feeds

Rss support for tags
Merge pull request #2370 from halfdan/2062-robots

Serve default robots.txt
Merge pull request #2393 from robgraeber/copyright-year

Updating copyright range
Update copyright