Moved standard charsets to a separate class

git-svn-id: https://svn.apache.org/repos/asf/james/mime4j/trunk@1482289 13f79535-47bb-0310-9956-ffa450edef68
Moved CodecUtil methods to ContentUtil / EncoderUtil

git-svn-id: https://svn.apache.org/repos/asf/james/mime4j/trunk@1482291 13f79535-47bb-0310-9956-ffa450edef68