v0.3.8

New features:

• Now you can use cancel to stop an active task of a task with auto_recrawl enabled.
• Handler.crawl_config will now be applied to the task when fetching. (It's applied when the task created before, that means proxy/headers can be changed afterward). See http://docs.pyspider.org/en/latest/apis/self.crawl/#handlercrawl_config

Fix several bugs:

• * Fixed a global config object thread interference issue, which may cause connect to scheduler rpc error: error(10061, '') error when all --run-in=thread (default in windows platform)
• Fix response.save lost when fetch failed issue
• Fix potential scheduler failure caused by old version of six
• Fix result dump return nothing when using mongodb backend