Clarify usage of the simpledb_polyfill.js library

[jeffposnick]

#47 adds in a local copy of a library that wraps IndexedDB, written by a Googler, Joshua Bell, and published at https://gist.github.com/inexorabletash/c8069c042b734519680c

I need to contact him and clarify whether he considers this code ready to be used, and if so, whether he has plans for officially releasing it somewhere other than in that gist, so that it could be pulled in as an official dependency.

There are a few other libraries wrapping IndexedDB we could use if necessary, but his is particularly small and straightforward, and works well for our needs at this time.

[jeffposnick]

I heard back from Joshua, and he has no plans on releasing this himself, but is fine with it being used. (Though he's worried about the lack of tests.)

wibblymat/shed#18 tracks whether this library can get rolled up into the shed project, which I think would be a good home for it. I'll keep this issue open until I hear back about that.

[jeffposnick]

Mat doesn't want to integrate simpledb into shed, so it looks like we'll just use our own local copy of it.