Adds HTTP Get rate limitation on cache

We don't want to have a large bandwidth on bedetheque.com, so the pace
at which we perform GET request is limited by a maximal number of
request over a window (per example no more than 10 request over 10
second)

If more request are required, the request is simply paused until we go
back within the limit ;). Go rulez !

album_description_cache_test.go

@@ -3,6 +3,7 @@ package main
 import (
 	"io/ioutil"
 	"os"
+	"time"
 
 	. "gopkg.in/check.v1"
 )
@@ -30,7 +31,7 @@ func (s *AlbumDescriptionCacheSuite) TestCanFetchCache(c *C) {
 	c.Assert(err, IsNil)
 	defer os.RemoveAll(tmpdir)
 
-	cache, err := NewAlbumDescriptionCache(tmpdir)
+	cache, err := NewAlbumDescriptionCache(tmpdir, 10, 10*time.Second)
 	c.Assert(err, IsNil)
 
 	for ID, expected := range data {