Extended new builder classes with methods to create copies of message elements and to parse message from content stream

git-svn-id: https://svn.apache.org/repos/asf/james/mime4j/trunk@1611278 13f79535-47bb-0310-9956-ffa450edef68
EntityBuilder -> /ParserStreamContentHandler; minor cleanups

git-svn-id: https://svn.apache.org/repos/asf/james/mime4j/trunk@1611279 13f79535-47bb-0310-9956-ffa450edef68
Added unit tests for AbstractEntityBuilder and BodyPartBuilder; incremental improvements in builder implementations

git-svn-id: https://svn.apache.org/repos/asf/james/mime4j/trunk@1617565 13f79535-47bb-0310-9956-ffa450edef68
Added typed #getField methods
Work in progress