add ability to export/import fs store

It'd be useful to be able to export or import the fs store for debugging purposes, so this changeset adds fs.importStore/exportStore functions (and export/import UI). The functions are pretty memory-inefficient, but they're good enough for debugging purposes. Export is assisted by the new FileSaver module, while the test relies on the new QUnit/equiv module.

> Oops, this |console.log| remained :smile:
…
> As a followup, could we show somehow that an import has finished?

I removed the *console.log* call and added an alert about the import in #775.