add basic .hgignore file for most common generated/temporary files
add basic .hgignore file for most common generated/temporary files

madlib/eigen_backup,rotorliu/eigen,madlib/eigen_backup,rotorliu/eigen,mjbshaw/Eigen,mjbshaw/Eigen,madlib/eigen_backup,rotorliu/eigen,rotorliu/eigen,madlib/eigen_backup,mjbshaw/Eigen,mjbshaw/Eigen