New str2saltedsha512 function for OS X Passwords

New str2saltedsha512 function for OS X Passwords

OS X 10.7 introduced salted-SHA512 password hashes as opposed to the
older LANMAN + SHA1 hashes. To assist in generating properly-formatted
password hashes, this commit adds the str2saltedsha512() function which
accepts a single string argument (the password) and returns a
salted-SHA512 password hash which can be fed as the password attribute
of a user resource in OS X 10.7.

Spec tests are also added to ensure that functionality isn't broken with
future commits.