Bug 801598 - Exposing OS.File communication mechanism as an independent API. r=froydnj

--HG--
rename : toolkit/components/osfile/modules/_PromiseWorker.jsm => toolkit/components/promiseworker/PromiseWorker.jsm
Bug 801598 - Making OS.File use the now public PromiseWorker API. r=froydnj
Bug 801598 - Making Thumbnails API use the now public PromiseWorker API. r=ttaubert
Bug 801598 - Making SessionWorker use the now public PromiseWorker API. r=ttaubert