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 #650 from Curita/dlhandler-disable

Allow to disable a downloader handler just like any other component