first prototype of the thumbnail service

Currently uses in memory caching and loads the file from the local
filesystem.

pkg/thumbnails/cache/cache.go

@@ -0,0 +1,11 @@
+package cache
+
+import (
+	"image"
+)
+
+// Cache defines the interface for a thumbnail cache.
+type Cache interface {
+	Get(key string) image.Image
+	Set(key string, thumbnail image.Image) (image.Image, error)
+}