API requests

Things we want the Gist API to have:

Fields

- modification date

Requests

- gists modified since… [handy for keeping gists in sync across devices/web]
- * call to list gists, incl. private gists
- real API for updating gists, instead of kludgy form posts
- Mark a gist as public or private (or at least make a private gist public)

Future

- gist network—forks n branches – super not necessary for 1.0