CrawSpider: support process_links as generator

Adding tests for CrawlSpider's process_links
Merge pull request #590 from Digenis/master

downloaderMW doc typo (spiderMW doc copy remnant)
add SEP-021 (Add-ons) - work in progress
fix a reference to unexistent engine.slots. closes #593
Merge pull request #594 from dangra/593-engineslots

fix a reference to unexistent engine.slots.
Add 0.22.2 release notes
DOC Use pipelines module name instead of pipieline following default project files.
Merge pull request #596 from darkrho/doc-pipeline

DOC Use pipelines module name instead of pipieline following default project files.
remove references to deprecated scrapy-developers list
TST Improved twisted installation in tox.ini for Python 3.3

Install twisted trunk because released twisted doesn't install properly,
see https://twistedmatrix.com/trac/ticket/6539.

Also, test-requirements are removed for 3.3 because mitmproxy doesn't
install in Python 3.3, and mock is in Python 3.3 standard library.

Note that trial scrapy is not doing what is expected because twisted doesn't
have trial runner for 3.3 yet, so this command will most likely call "trial"
outside tox virtualenv and will use incorrect interpreter.
DupeFilter: add setting for verbose logging + stats counter for filtered requests
Remove _log_level attribute as per comments
Fixed #441.
Merge pull request #555 from redapple/crawlspider-processlinks

CrawlSpider: support process_links as generator
Merge pull request #553 from redapple/dupefilter-verbose

DupeFilter: add settings for verbose logging and filtered requests stats
fixes dynamic itemclass example usage of type()
Merge pull request #603 from barraponto/dynamic-itemclass-docfix

fixes dynamic itemclass example usage of type()
rename attribute to match conventions used for XXX_DEBUG settings (in autothrottle and cookies mw)
remove no longer used setting: MAIL_DEBUG
Merge pull request #602 from Blender3D/email

Added a mimetype parameter to `MailSender.send`