Update Mention regex to include att [Rr][tT] variations

This commit is actually based on a pull request for the conformance
suite.
https://github.com/twitter/twitter-text-conformance/pull/58
As shown there twitter actually recognizes a mention no matter
what variation of "RT/rt/Rt/rT" preceeds it.
This commit was done on the dime of Simply Measured simplymeasured.com