DownloaderMiddleware docs: Update process_request, proper explanation of IgnoreRequest.

Also:
* Change terminology to eliminate uses of terms such as "request middleware" to refer to the process_request methods of installed middleware.
* Remove description of "immediate redirection", as it is misleading.

Further changes.
Modify ItemLoader to support XPath and CSS selectors

Deprecate XPathItemLoader (now an alias to the new ItemLoader)
Merge pull request #461 from redapple/selectorloader

Add "unified" SelectorItemLoader (supports .add_css() and .add_xpath())
travis-ci updated to pypy 2.2
Merge pull request #425 from audiodude/master

DownloaderMiddleware docs: Update process_request and minor cleanups.
include_package_data is required to build wheels from published sources
Add 0.20.1 release notes

Conflicts:
	docs/news.rst
fix method name in tutorial. closes GH-480