Moved common MIME entity building logic to a separate class

git-svn-id: https://svn.apache.org/repos/asf/james/mime4j/trunk@1606454 13f79535-47bb-0310-9956-ffa450edef68
Added builder class for BodyPart and Multipart instances

git-svn-id: https://svn.apache.org/repos/asf/james/mime4j/trunk@1606455 13f79535-47bb-0310-9956-ffa450edef68