Support case-insensitive domains in url_is_from_any_domain()

fix scrapy.version_info when SCRAPY_VERSION_FROM_GIT is set

py.test replaces AssertionError exception with its own subclass in an import hook.
For some reason when top-level shortcuts were added "except AssertionError" in
contracts.__init__ meant "except py.test AssertionError" while scrapy.exceptions.ContractFail
was still a subclass of a builtin AssertionError, so Contracts reported errors
instead of failures and this made tests in test_contracts.py fail.

For now - just disable fancy py.test asserts.

[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