Vendor base64.js for encoding support

Encoding library base64.js written by Fred Palmer and released under the
MIT License.
Implement import/export for backups

The 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.)

Switch 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!
Merge pull request #15 from Pringley/base64save

Well done. I've merged and will test tonight. I am a little worried about Chrome's 2.5MB `localStorage` limitation, but I suspect we'll be well under that. Thanks again for the commit. Your base64.js is MIT, right?