Expose IndexedDb api (bug 786688)

Compared to previous commit (862ba11a):

* remove db prefixing
* per-addon security principal

Risks:

* will make data created using previous sdk INACESSIBLE
  (unless they take effort to find the data and export it,
  which is non-trivial)
* relying on `self.packageID` might make data INACCESSIBLE in the
  future if we change how `packageID` is derived.

Tests pass.