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. 
>  Because I'd rather incure unnecessary computations than API changes ;)
I'd really not. I'm already on a tight computational budget with tablets here and I can't afford to hit the CPU ceiling for my PhD again (as in the Java experiments) just so that the API looks nice for hypothetical people who don't get 4x4 matrices or quaternions. Please remember that the first and foremost stakeholder (and in my opinion unfortunately the only real one for the moment) is the Chili projects and specifically for the camera transformations the only stakeholder for the moment is me. So I think you would understand when I put my project before trying to make this into a flawless open source project for everyone.