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.
duplicate 'use' in line 87
Merge pull request #460 from tntC4stl3/master

duplicate 'use' in line 87
telnet client: fix unexisting reference to engine.slots
Merge pull request #464 from kalessin/telnet

telnet client: fix unexisting reference to engine.slots
Deprecate body_or_str helper function only used by xml iterators
Add note to item-pipeline documentation explaining order in the ITEM_PIPELINES setting.
Do not call body_as_unicode on non text responses. closes #462
lxml is required, no need to skip tests.
add a tox env for Python 3.3
Elaborate on use of order numbers
Merge pull request #465 from bjlange/master

Add note to item-pipeline documentation explaining order
allow to use settings for defining http error handling defaults
Merge pull request #466 from kalessin/httperror

allow to use settings for defining http error handling defaults
better names for HANDLE_* settings, and added doc
Modify ItemLoader to support XPath and CSS selectors

Deprecate XPathItemLoader (now an alias to the new ItemLoader)
Remove "x-gzip" from Requests' "Accept-Encoding" header
Merge pull request #469 from redapple/xgzip

Remove "x-gzip" from Requests' "Accept-Encoding" header
Merge pull request #461 from redapple/selectorloader

Add "unified" SelectorItemLoader (supports .add_css() and .add_xpath())
travis-ci updated to pypy 2.2