Add some more alternative asm implementations, for reference.

From one of their NIST CD submission packages.
Fix the assembly implementation, use it!
Disable use of the assembly impl on Darwin, it doesn't build there.
Use _xmm32 asm implementation for 32bit machines.
Reported to be *quite* a bit faster.
Fix skein_block_xmm32, 32bit seems to work correctly now.
hash.c: Add vanity prefix/suffix support, improve search logic.

More searching.
Use more double-friendly goal for throughput calculation.
Quick code cleanup, commenting.

clang-format'd, too.
Ensure we flush after printing important information (found a new best).
Add comment regarding popcnt implementation.
hash.c: More cleanup, documentation.

Only important functionality change is
that the <num_threads> argument is now optional,
and if unspecified defaults to the number
of online processors per sysconf.
Move documentation into README.md for github.
Merge branch 'master' into arc4random

Conflicts:
	Makefile
	hash.c