New str2saltedsha512 function for OS X Passwords

[glarizza]

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.

[jeffmccune]

I don't think it's a good idea to use is_a?(String) here. We might pass in something that's a kind of String like object but isn't a String itself.

It'll probably be more dynamic if we just use the object's to_s method to convert it to a string representation, whatever the heck it is.

The spec tests should then cover our expectations about how different input objects behave.

[jeffmccune]

Actually, on second thought the explicit catching of the String is probably a better UX if the user accidentally passes in a Hash or Array.