Update QtCreator to 2.8.1, add Qt 5.1.1 and QtSDK

Because other people seems to be interested in a Qt 5 package, you can find here my work on the packaging of Qt 5.1.1 for nixpkgs. Technical descriptions are present in each related commit.

As my mesa change will not affect the functional behavior of its current dependencies, I am asking you to directly integrate it into the master branch. You can still push it into x-updates if the necessary mesa and dependencies rebuild on hydra remains an issue for you.

Because I did not had the time yet to write a dedicated wiki page, feel free to ask me anything about programming using Qt 5 and QtCreator and QtSDK (a pre-packaged Qt+QtCreator) on NixOS :wink: