Use wp_hash_password and wp_check_password

Outsource password hasing to PHPass, in other words. `wp_hash_password`
and `wp_check_password` are also pluggable -- let's end users determine
their own level of hash strength rather than just a SHA hash.
Merge pull request #1 from chrisguitarguy/use_wp_password

Use wp_hash_password and wp_check_password