Add better remote control to push-to-trunk auto-roll script.

This CL enables controlling the automatic push-to-trunk via a settings file .auto-roll in the home directory or via the v8 status app message.

Pushes can be disabled by setting .auto-roll to {"enable_auto_roll": false} or by adding the phrase "nopush" or "no push" to http://v8-status.appspot.com/.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/113973003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18472 ce2b1a6d-e550-0410-aec6-3dcde31c8c00