Add graph segmentation using random walks

This is an implementation of the algorithm described in "Random Walks
for Image Segmentation" by Leo Grady which works with BGL graphs.
Add unit tests for random walker segmentation
Add ${EIGEN_VERSION} to FindEigen.cmake
Add guards to check Eigen version

Random walker needs Eigen::Sparse module, which is available since
version 3.1.0.
Add custom PPA to get latest Eigen for Travis
Merge pull request #407 from taketwo/master

Add graph segmentation using random walks