Implement import/export for backups

![screenshot 2013-10-25 09 16 48](https://f.cloud.github.com/assets/633203/1407910/cf8c6612-3d77-11e3-9a74-0c703b662256.png)

The proposed save format is base64-encoded JSON. This makes the tamper potential less obvious to non-technical users. (Technical users can already just inject javascript to make the game do what they want, so there's little reason to use actual encryption on the save files.)

Also, I switched the import/export display to a textarea instead of an alert, because alerts on some platforms have maximum lengths less than the required size of the save data, which causes corruption!

Finally, Fred Palmer's open source base64.js is used for encoding/decoding the save files.