use a proper cache

e.g. lru, written by google/go people and it's hopefully going to
protect us from consuming too much memory.

go/favicon.go

package main
import (
	"code.google.com/p/cascadia"
	"code.google.com/p/go.net/html"
	"errors"
	"fmt"
	"net/http"
	"net/url"
	"os"
	"sync"
	"code.google.com/p/cascadia"
	"code.google.com/p/go.net/html"
	"github.com/golang/groupcache/lru"
)
var faviconCache = make(map[string]string)
var faviconCache = lru.New(10000)
var lock sync.RWMutex
func main() {
		host = parsed.Host
	}
	lock.RLock()
	faviconUrl, cached := faviconCache[host]
	fu, cached := faviconCache.Get(host)
	lock.RUnlock()
	if cached {
		return faviconUrl, nil
		return fu.(string), nil
	}
	faviconUrl, err = GetFavicon(u)
	faviconUrl, err := GetFavicon(u)
	if err != nil {
		return faviconUrl, err
	}
	lock.Lock()
	faviconCache[host] = faviconUrl
	faviconCache.Add(host, faviconUrl)
	lock.Unlock()
	return faviconUrl, nil
}