v2.0 release

• add support for multithreading (e3ac559)
• greatly improve status display on runtime (33a4f98)
• make now builds with optimizations (like old make release)
• add support for multiple debug levels (make debug level=N, closes #3)
• fix bug in hmap load factor calculation (7a4b304, closes #15)
• fix potention division by zero (00def6f)
• add error handling for 'no space left on hashmap' (e1dfedf)
• update C-Thread-Pool to version 25fa679-2020-07 (a316b04)
• update LICENSE (7f08d0f)
• ignore -Wimplicit-fallthrough on build (closes #16)