ENH: Update Eigen for clang c++11

Create a mercurial--git bridge for eigen

http://felipec.wordpress.com/2012/11/13/git-remote-hg-bzr-2/

git config --global remote-hg.hg-git-compat true
git remote add eigenHG  hg::http://bitbucket.org/eigen/eigen
git fetch eigenHG
git push https://github.com/BRAINSia/eigen.git eigenHG/master:master -f