Make CURL optional at build time

example:
    make EXTRAFLAGS="-DHAVE_CURL=0" LIBFLAGS=""
Fix #7 - Add option-parsing

* Options:
  -h, --help : usage
  -H, --hash : change hash to target
  -B, --benchmark : Run for approximately this many iterations
  -t, --trials : benchmark trials
  -T, --threads : number of concurrent threads

* Benchmark and find optimal profile for defaults (-O3, unroll=10)
* Update README

* If nthreads == 1, skip pthreads
Remove block_bench (redundant)