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 (as of now, github issues broke down and you may not be able to see it but it is the flipping issue).

For now, the quick & dirty covariance calibration seems to work decent enough. Please test further.

A huge issue now is gimbal lock. We need quaternions to fix this.

In the near future, more goodies such as 

- Calibration API
- Enable/disable on demand API
- Quaternions
- IMU integration
- Optimizations
- Sample

will follow.