CMake finds correct Eigen config in Ubuntu 14.04.

The installation tutorial instructs to install Eigen3.  A standard
Ubuntu installation has no FindEigen.cmake, but only FindEigen3.cmake.
Therefore, I instructed CMake to require Eigen3 instead of Eigen.
Compatibility update for ROS Indigo.

CvBridge does not have the imgmsg_to_cv function anymore, but it is
renamed to imgmsg_to_cv2.
Merge branches 'indigo-compatibility' and 'eigen-ubuntu' of github.com:pkok/kalibr