Optimized engine for reaching 32768

Lookahead now depends on previous evaluation, which allows for a more rapid increase of the lookahead if something is going wrong.
The custom hashmap now avoids more collisions because of a better hash function and more (512 MB) memory.
Fixed: score measuring function would overflow with tiles of 65536.
Removed unused code (sampling and simple strategy).
Merge branch 'master' of https://github.com/mmiermans/solver2048.git