fixed namespace pollution ("using namespace Eigen/std" in headers")

Thanks for sharing this useful library.

There is a problem with namespace pollution I ran into when using it in conjunction with another library that does not use Eigen but defines its own Matrix / Vector classes: Sophus has a few "using namespace Eigen" in its headers, which leads to conflicts.

This is also mentioned as item 59 in Alexandrescu's "C++ Coding Standards"
http://www.gotw.ca/publications/c++cs.htm

The commit below fixed this for me.