Implement Qiniu storage

Related issue #5

@lepture @tonyseek plz review, thx!
@irachex I misunderstood. Sorry, please ignore it.

Could we override `qiniu.conf.ACCESS_KEY` and `qiniu.conf. SECRET_KEY ` with current instance's configuration before all methods be called? It's dirty, I know. >_<

It just like the `_req_ctx_stack` of Flask. The current context (configuration) is always on the top.