add basic caching support

includes two implementations, a no-op NopCache and an in-memory
MemoryCache.

imageproxy.go

@@ -1,16 +1,26 @@
 package main
 
 import (
+	"flag"
+	"fmt"
 	"log"
 	"net/http"
 
+	"github.com/willnorris/go-imageproxy/cache"
 	"github.com/willnorris/go-imageproxy/proxy"
 )
 
+var port = flag.Int("port", 8080, "port to listen on")
+
 func main() {
+	flag.Parse()
+
+	fmt.Printf("go-imageproxy listening on port %d\n", *port)
+
 	p := proxy.NewProxy(nil)
+	p.Cache = cache.NewMemoryCache()
 	server := &http.Server{
-		Addr:    ":8080",
+		Addr:    fmt.Sprintf(":%d", *port),
 		Handler: p,
 	}
 	err := server.ListenAndServe()