Update gist posting to work with the lastest changes to gist.github.com

[nilbus]

Using a more generic regex to find the gist id. The old regex was failing because github no longer considers this HTTP request to be logged in, despite having the API token, so it does not provide an edit button.

Github now expects the parameter action_button=private, instead of private=true. All gists were being created as public gists.

The "TODO: Replace true with private preference" comment was outdated, since this has since been done.

Note: This patch does not require GitX to be rebuilt. The updated history.js file can be dropped into an existing GitX installation at /Applications/GitX.app/Contents/Resources/html/views/history/history.js and it will go into effect after restarting GitX. The GitX download for SciMed/gitx contains this patch.