Use the FindEigen3.cmake from the Eigen repository

Downloaded from [1] .
The used  `FindPackageHandleStandardArgs` CMake module is available since CMake 2.6 [2], that is already the minimum CMake version required by orocos_kdl. 

The old `FindEigen3.cmake` script is not compatible with Windows, this one should work with Windows and also cover the old use cases. 

[1] : https://bitbucket.org/eigen/eigen/src/0c883fcd96da1bcfc697bb06a421ad567689a8b2/cmake/FindEigen3.cmake
[2] : http://www.cmake.org/Wiki/CMake_Version_Compatibility_Matrix/StandardCMakeModulesFindL#cite_ref-19