Use the FindEigen3.cmake from the Eigen repository

Downloaded from https://bitbucket.org/eigen/eigen/src/0c883fcd96da1bcfc697bb06a421ad567689a8b2/cmake/FindEigen3.cmake .
The used  `FindPackageHandleStandardArgs` CMake module is available since CMake 2.6
Get updated FindEigen3.cmake from Eigen repository

For more information check the relevant Eigen PR: https://bitbucket.org/eigen/eigen/pull-request/76/findeigen3cmake-add-reading-hints-of-eigen/diff
Merge pull request #29 from traversaro/patch-3

Use the FindEigen3.cmake from the Eigen repository