Update httperror.py

Log ignored HTTP pages when the response code is not allowed or not configured to be handled.
New doc: clickdata in Formrequest.from_response

Documentation about:
 * clickdata parameter in Formrequest.from_response
 * nr attribute in clickdata dict
 * default behaviour when clickdata is None
New tests: clickdata's nr in Formrequest.from_response
FormRequest doc improvements

Clickdata doc enhancements:
 * Fix xml attributes mention
 * nr is 0-indexed reference
More appropriate assert in FormRequest test
Alow to disable a downloader handler just like any other component.
Tests for loading download handlers
Fix minor typo in DownloaderHandlers comment
Doc for disabling download handler
Minor fixes in LoadTestCase in test_downloader_handlers

 * same __init__ parameters in both download handlers's mocks
 * additional assertion in test_disabled_handler
Merge pull request #646 from Curita/formrequest-clickdata-nr-index-tests

Tests for clickdata's nr index in FormRequest
Merge pull request #645 from Curita/formrequest-clickdata-doc

Documentation for FormRequest.from_response clickdata parameter
Merge pull request #650 from Curita/dlhandler-disable

Allow to disable a downloader handler just like any other component
add message when raise IngoreReques; fix item_scraped document
Merge pull request #654 from tpeng/patch-2

add message when raise IngoreReques; fix item_scraped document
Merge pull request #612 from deed02392/patch-1

Update httperror.py
reduce code duplication in test_spidermiddleware_httperror
scrapy.utils.test.docrawl function
Fix for #612 + integration-style tests for HttpErrorMiddleware
TST fix file descriptor leak and a bad variable name in get_testlog