Create SET cache endpoint

Cache SET endpoint implementation

Closes #3 (closed) FYI: @tziai @valerii.kalashnikov

design/design.go

@@ -64,7 +64,7 @@ var _ = Service("cache", func() {
 	})
 
 	Method("Set", func() {
-		Description("Set value in the cache. The HTTP request body is stored as value and the key is assembled from HTTP request headers.")
+		Description("Set a JSON value in the cache.")
 
 		Payload(CacheSetRequest)
 		Result(Empty)