Parameters to Request() in wrong order

Implied that callback wasn't the first optional unnamed parameter.
Move callback blob; mention errback
Merge pull request #426 from scrapy/selectors-unified

[MRG] Selectors unified API
Merge pull request #226 from scraperdragon/patch-1

Parameters to Request() in wrong order
typo fix in TextResponse docs
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.