added magicfields middleware
replaced proxy.scrapinghub.com with proxy.crawlera.com
improve ban implementation
Merge pull request #16 from nramirezuy/hubproxy/bans-per-slot

improve ban implementation
implementation retry-after
Merge pull request #17 from nramirezuy/hubproxy/retry-after

implementation retry-after
Merge pull request #12 from kalessin/xfields

Magic fields middleware