Some encryption fixes:
- don't rely on SecretKey object having the right algorithm set
- leave encryption-description parsing of string/stream to xmlbeans and refactor it to one location
- use namespaces of schema instead of hard-coded strings
- use CryptoFunctions.getMessageDigest() instead of code duplication

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1588874 13f79535-47bb-0310-9956-ffa450edef68