Update leaks.rst: there is now only a single spider in a process.
DOC bring back notes about multiple spiders per process because it is now documented how to do that
DOC request queue memory usage
drop support for CONCURRENT_REQUESTS_PER_SPIDER
RobotsTxtMiddleware: remove unused attribute
dont_obey_robotstxt meta key; don't process requests to /robots.txt
don't hide original exception in scrapy.utils.misc.load_object
DOC document from_crawler method for item pipelines; add an example.
Merge pull request #904 from scrapy/from_crawler_docs

[MRG] DOC document from_crawler method for item pipelines
Merge pull request #902 from scrapy/load_object_full_traceback

[MRG] scrapy.utils.misc.load_object should print full traceback
Merge pull request #896 from scrapy/robotstxt-once

[MRG] process robots.txt once
Merge pull request #894 from kmike/leaks-docs

Leaks docs
mark SEP-019 as Final
DOC typo fix in leaks.rst
updated deprecated cgi.parse_qsl to use six's parse_qsl
Merge pull request #909 from VKen/master

updated deprecated cgi.parse_qsl to use six's parse_qsl
Drop old engine code

  * remove Downloader import unused since 1fba64
  * remove CONCURRENT_SPIDERS deprecation warning from a1dbc6 (2011)
Merge pull request #911 from nyov/nyov/dev

Drop old engine code
Merge pull request #895 from scrapy/we-are-past-0.15

[MRG] drop support for CONCURRENT_REQUESTS_PER_SPIDER
DOC document download_timeout Request.meta key and download_timeout spider attribute.