properly treat utf8 chars in password

utf-8 encoding used to be bundled with the SHA256 library. However we
only want to utf8 encode strings that are actual user input. We do not
want to encode the bytearrays that are used when hashing the hex values
calculated during for SRP.

So I separated the utf-8 encoding and the sha256 hashing.
Merge pull request #4 from azul/bugfix/utf8-in-sha

properly treat utf8 chars in password