Persist data to gists

[djw]

Here's the beginnings of a pull request which persists (up)loaded data to gists.

• [x] Persist data when it's first loaded (not as part of normal autosave)
• [x] Load persisted data from gist
• [x] Fix representation in files section of data package

[djw]

Also, we need to decide if we update the persisted data when it's altered by a script.

Pro:

• We wouldn't have to run the script for the visualisations to work, see #88

Con:

• We'd be running scripts against pre-processed data. This shouldn't matter if scripts are idempotent.

[rufuspollock]

I thought the decision is to store the updated data.

[djw]

@rgrp Could you check you're happy with the serialisation in:

https://gist.github.com/djw/6d90c64c2981ff7d656a#file-datapackage-json

[rufuspollock]

Looks good - we should do #98 (assigned to me but you should just dive in!)