Added unit tests for MultipartBuilder and MessageBuilder

git-svn-id: https://svn.apache.org/repos/asf/james/mime4j/trunk@1618035 13f79535-47bb-0310-9956-ffa450edef68
Removed mutators from DOM interfaces (Message, BodyPart, Multipart, etc) that are now provided by corresponding builders

git-svn-id: https://svn.apache.org/repos/asf/james/mime4j/trunk@1618036 13f79535-47bb-0310-9956-ffa450edef68
Moved Message, Header and Entity implementation test cases

git-svn-id: https://svn.apache.org/repos/asf/james/mime4j/trunk@1618037 13f79535-47bb-0310-9956-ffa450edef68