eigen: initial recipe contributed by Kartik Mohta (resolves #71)

This commit adds the eigen library from Kartik Mohta's OpenEmbedded
layer. Acknowledgements are added in the README file.
tuned spacing
catkin.bbclass: make ROS_BP composed from ROS_BPN and version

The new definition was the original intention, and the originally
wrong definition happened to slip in.
Initial commit and discussion about the intention is in commit
c6837ecbf8d1ea8bd46d59dd3a2832d9b69faaab.
Merge pull request #75 from bulwahn/stable

redistribution of eigen and minor tuning