Make automatic support for private-browsing explicit. re #4

In the 'per-window private browsing' world, the right solution for
recording data is no longer obvious.  Addons need to make decisions
about this for themselves.
moved old indexed-db
Support for FX 17.  fix #3

17 is support IFF one includes indexed-db-17.
18+ is supported fine on both mobile and desktop.

Udpated docs.
Version 0.3, removed private-browsing automagic, indexeddb