Add crypt-sha512 utility; minor fixes

Some changes I hope you'll like:
* Fixed 'require "../lib/unix_crypt"' so it uses File.expand_path, which makes it work on more Rubies
* Added crypt-sha512 command-line utility (no additional dependencies required to use except base Ruby)
* Added homepage, license to gemspec
* Added authorship, contributors to readme
* bumped version.

Thanks for creating this gem! I've been using it to create encrypted passwords for virtual machine images for Packer.