media配置

(1) media配置：
针对：
avatar = models.FileField(upload_to='avatars/', default="/avatars/default.png")
avatar = models.ImageField(upload_to='avatars/ ', default="/avatars/default.png")
MEDIA_ROOT=os.path.join(BASE_DIR,"blog","media"

(2) MEDIA_URL="/media/"
# media 配置
url(r'^media/(?P<path>.*)$', serve, {'document_root': settings.MEDIA_ROOT}),