[MRG] Resolved issue #546. Output format parsing from filename extension.

[MRG] make scrapy.version_info a tuple of integers

Docs build and linkcheck in tox env

Run tests with pytest instead of trial

Fixed XXE flaw in sitemap reader

  * Change current dir to tmp dir on each test run
  * Log twisted with test.log

[WIP] try to restore pypy tests

Fixed default value of attrs argument in SgmlLinkExtractor to be tuple

Fixed SgmlLinkExtractor constructor to properly handle both string and list parameters (attrs, tags, deny_extensions)

Added content-type check as per issue #193

[MRG] Fix for #612 + small tests cleanup