Democracy 1 - 0 Dictatorship

After huge thread on eigen mailing list, it appears that i'm the only one in favor of prefix Ei. Everybody else prefers namespace Eigen like we did in Eigen 1. So, revert.

Also add a macro USING_EIGEN_DATA_TYPES that application programmers can use to automatically do "using"on the Matrix class and its matrix/vector typedefs:
using Eigen::Matrix;
using Eigen::Matrix2d;
using Eigen::Vector2d;
... (the list of typedefs is really long).

thanks to the suffixes, the Vector typedefs aren't really polluting.

P.S. Danny, please skip this one :) I know you already reported the namespace->prefix move, now that one would be too much noise :)