Bug 868928 - Expand bindings/README's section about context data. r=jbeatty

Expand README's section about context data to show the $var syntax and
data-l10n-args
Localized
Pass reference to the context to Localized
Localize.prototype;  add relocalize
Expose handler.extend
collectIds
Simplify function names
Remove initializeDocumentContext
l10n (passed to localize's callback) now has the reason (fixes bug 868696)
Remove Localized
Add bindLocalize so that localize doesn't have to take a 3rd arg
Don't extend the localizationHandler on the initial localization of document.body
Don't allow duplicate items in idsOrTuples
gandalf's patch to patch
Remove global nodes;  remove RetranslationManager::rebindGet
Remove HTMLElement.prototype.localize
bound.extend returns many
remove onDocumentReady; s/onLocalize/localizeBody
Remove data-l10n-args from bindings/README