Added hashSync overload that accepts rounds instead of salt instance

[bookercodes]

Outline I added an overload to the hashSync function that accepts the number of rounds to use.

Benefit Instead of having to create a salt instance and pass it to the hashSync function:

var salt = bCrypt.genSaltSync(12);
var hash = bCrypt.hashSync('super secret', salt);

Consumers can now supply the number of rounds directly, as an argument:

var hash = bCrypt.hashSync('super secret', 12);

This is similar to how node.bcrypt.js works.

Todo

• [ ] It would be wise to write a test that uses a spy to verify that the specified number of rounds are being used and not just the default.
• [ ] Document the overload in README.md.
• [ ] Document the overload in the hashSync comments.
• [ ] Bump version number

I am happy to do all these things, I just want to check that this is something that you would actually consider merging first.

Thank you!