added mac support; build succeed on mac, but may break Linux(haven't tested yet)
Added missing import... I knew it would break Linux support!!
fixed the bug that ReadPassword would return empty string; That was because copy() does not expand space in destination slice, but just copies the minimum of len(src) and len(dst) (http://golang.org/pkg/builtin/#copy)
Merge pull request #4 from songgao/bugfix-ReadPassword-copy-to-nil

Bugfix read password copy to nil