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.

> Nit: logging behing DEBUG_FS?

Yep, c4b3b49.