Change hash_password to match django hash string

I changed hash_password output to include algo, salt, iteration.
Making it match with django hash format.

From the example in README:

```ruby
Pbkdf2PasswordHasher.hash_password(pass,salt,it) #=> "uDAu+fkRHoZk03PKp0bzrXDWc4j4mhkzGBm7ljbvp58="
```
Pbkdf2PasswordHasher.hash_password(pass,salt,it) returning `uDAu+fkRHoZk03PKp0bzrXDWc4j4mhkzGBm7ljbvp58=` which is not the one check_password take in.

I changed it to
```ruby
Pbkdf2PasswordHasher.hash_password(pass,salt,it) #=> "pbkdf2_sha256$1000$NaCl$uDAu+fkRHoZk03PKp0bzrXDWc4j4mhkzGBm7ljbvp58="
```

I have updated the spec and README as well.

Lincoln