* Use microsecond value in POSIX `sys/time.h` to seed `srand()`,
  theoretically giving different output upon each execution (using
  regular UNIX time, the output only changes every second)
* Add `--use-posixtime` feature in configure script to automatically
  compile with necessary macros
* Only use upper bits of `rand()` as lower bits are unreliable
* Update documentation to reflect changes

See merge request !3