Scrapy-crawlera

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- Use request.meta than response.meta in the middleware

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- Consider Crawlera response if contains `X-Crawlera-Version` header
- Build the documentation in Travis CI and fail on documentation issues
- Update matrix of tests

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- Added more stats to better understanding the internal states.
- Log warning when using `https://` protocol.
- Add default `http://` protocol in case of none provided, and log warning about it.
- Fix duplicated request when the response is not from crawlera, this was causing an
infinite loop of retries when `dont_filter=True`.

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- Enable crawlera on demand by setting ``CRAWLERA_FORCE_ENABLE_ON_HTTP_CODES``

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- Remove username and password from settings since it's removed from crawlera.
- Include affected spider in logs.
- Handle situations when crawlera is restarted and reply with 407's for a few minutes
by retrying the requests with a exponential backoff system.

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- Correctly check for bans in crawlera (Jobs will not get banned on non ban 503's).
- Exponential backoff when crawlera doesn't have proxies available.
- Fix ``dont_proxy=False`` header disabling crawlera when it is enabled.