FIX: Manifest validation false positive for type: "web" (fix #351)

- valid app types: web, privileged, certified
- permission table appStatus lookup: app, privileged, certified

If it isn't certified or privileged, it's appStatus "app":

https://hg.mozilla.org/releases/mozilla-b2g18/file/d9278721eea1/dom/apps/src/PermissionsTable.jsm#l413
Merge pull request #354 from rpl/fix-validation-type-web

FIX: Manifest validation false positive for type: "web"