Add a simple RESTful JSON API for card sets

Examples:

$ curl localhost:8080/sets/
{
  "colors.json": "Color quiz"
}

$ curl localhost:8080/sets/colors.json
{
	"name": "Hebrew: Days of week",
	"cards": [
		{ "answer": "yellow", "description": "What color is the sun?" },
		{ "answer": "blue", "description": "What color is a cloud?" }
	]
}

$ curl localhost:8080/sets/colorZ.json
HTTP/1.1 404 Not Found
...
{
  "message": "Card set not found!"
}
Add menu to choose card set
Change handling of a path

Very insignificant change.
Merge branch 'serverside-data'