Hides annoying global announcement messages from Amazon, on every MTurk page except the Dashboard (so you can still be aware of what's going on). Includes the HIT promotion messages
introduced on Feb 9, 2015, and the timechange/maintenance message as of how it was in Oct-Nov 2014 (previously hidden by this
one-time-use script). Does NOT hide normal MTurk messages about HITs no longer being available, captcha failures, and important stuff like that (some other blocking methods such as the AdBlockPlus browser extension may undesirably include those in the hiding). I will try to keep this updated if/when new variations of annoying messages appear in the future.
See also: my MTG thread post
For many more MTurk-related userscripts, see the Turking Scripts set