Add paginated version of the API, bumping to v2.

/api/v2/gists now accepts a `page` parameter and returns only 25 gists
at a time. To aid navigation for API clients, the response now has three
properties:

* `total_pages`: the total number of pages available;
* `page`: the current page number;
* `gists`: an array of the gists on the current page.