Kalman filter prototype

This PR is an early preview of Kalman filtering to 6D tag pose. Comes with hardcore c++11 `std::map::emplace()` and related syntax :)

Is a partial solution to #19.

For now, the quick & dirty covariance calibration seems to work decent enough. Please test further.

~~A huge issue now is gimbal lock and discontinuities in angle. We need quaternions to fix this.~~ done

In the near future, more goodies such as 

- ~~Calibration API~~ done
- ~~Enable/disable on demand API~~ done
- ~~Quaternions~~ done
- ~~IMU integration~~ done, see also https://github.com/chili-epfl/qml-imu
- ~~Optimizations~~ pretty much done
- ~~Sample~~ done
- ~~Remove `Filter` completely~~ done
- ~~Add tag discard mechanism~~ done
- ~~Fully document all math involved~~ done

will follow. 
>If you use chilitags to measure the move of one camera for example, this output is not usable as input.

I tried that by the way in https://github.com/chili-epfl/cellulo, either I failed to do it properly or it's not suitable for a handheld camera. This remark was also made by Stéphane Magnenat of Thymio who did it with feature tracking on image tags. The solution to this is visual/depth odometry, partially inertial odometry or any kind of solid odometry.