Make Pbkdf2 class API more flexible ( + 2 minor fixes)

Hi Robin!

## What's new

I've updated the `Pbkdf2` class to accept Bytes (`List<int>`) data as arguments.

## Why ?

I had an issue with hexadecimal token in Strings, the Pbkdf2.generate was incorrect.
So I've created a method `generateFromString` that wrap the `generate` function.
Now, I give the Bytes value of my hexadecimal string (salt) and the result is correct.



#### Improvements :
- Make `generate` return `List<int>`, not `String` to match `crypto` official package API ?