fix a possible error in contracts post hook
command with multiple crawlers, fix check command
disable logging for scrapy check
default to multi crawler commands

compatibility for old single crawler process
deprecated command's `crawler` property
logs about settings only shown once
port all scrapy commands to new CrawlerProcess
fix some missing logs with new crawlerprocess
fix log intermixing in scrapy shell command
remove redundant kwarg
don't start reactor if no crawler is scheduled
Merge pull request #330 from alexcepoi/contracts_fix