allow approximation of euclidian distances by lat/lon components
fix include order
mega-patch:
- fix regression that turned tree into binary search
- use C++11 emplace and thread local facility
- sort out includes
- use much faster approximations to save on trigonometry
- use higher fan-out but make leafs a little more compact
- replace raw C-style array by C++11's std::array
revert:

- GCC <4.8 does not support thread_local keyword.
- sticking with boost::thread_specific_ptr<> for now