Added support for HEAD requests
Bug 874070 - Fix selection tests and tab-browser lib to listen for load events on <browser> rather than <tab>
Merge pull request #1002 from gijsk/bug874070

fix Bug 874070 - SDK tests shouldn't check for <tab>'s load event but for browser's r=@zer0
Merge pull request #1004 from bkase/bug643156

Bug 643156 - Added support for HEAD requests r=@ochameau
Fix regression that removed `addon` global.

- Write tests to catch such regressions in a future.
Merge pull request #1005 from Gozala/bug/addon-global@874152

Bug 874152 - Fix regression that removed `addon` global.
Bug 850367 Page worker follows redirect, refactor match patterns for page-mods