set the exit code to non-zero when contracts fails

Elaborated Request priority value in Scrapy docs

grammatical issue

Per-spider settings and api cleanup: sep#19 update

[MRG] Fix engine to support filtered start_requests

Dupefilter request_fingerprint test and docs

fixed typo

changed 3 instances

Fix PEP8 warnings in project and spider template files

Added docs response selector shortcuts

Use response.xpath(), response.css() or response.selector instead.

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.