add two downloadmiddleware:GoogleCacheMiddleware and RotateUserAgentMiddleware

# GoogleCacheMiddleware:

<pre>
      this is a downloadmiddle to avoid getting banned,you can set the 
GOOGLE_CACHE_DOMAINS variable or you can set the user_agent_list 
attribute in your spider to define what domain you will use to visit the 
google cache,it is a list,eg:GOOGLE_CACHE_DOMAINS = ['www.woaidu.org',]
</pre>
# RotateUserAgentMiddleware:
<pre>
      this is also a downloadmiddleware to avoid getting banned,you can 
set the USER_AGENT_LIST in settings,then the middleware will random
 choose one of them as the user-agent,if you don't define it,then it will 
use the default user-aget,it contains chrome,I E,firefox,Mozilla,opera,netscape.
</pre>
# how to use them:
## for GoogleCacheMiddleware:
<pre>
         add "scrapy.contrib.downloadermiddleware.google_cache.GoogleCacheMiddleware":50
 in your DOWNLOADER_MIDDLEWARES,and define GOOGLE_CACHE_DOMAINSin your 
settings,eg: ['www.woaidu.org',]
</pre>
## for RotateUserAgentMiddleware:
<pre>
       add 'scrapy.contrib.downloadermiddleware.useragent.UserAgentMiddleware': None,
'woaidu_crawler.contrib.downloadmiddleware.rotate_useragent.RotateUserAgentMiddleware'
:400, in your DOWNLOADER_MIDDLEWARES.
</pre>