Switch from using the mime4j charset list to the Java 1.5 java.nio.charset Charset class.

Based on this comment in MIME4J, this seems like a win:


 * Utility class for working with character sets. It is somewhat similar to
 * the Java 1.4 <code>java.nio.charset.Charset</code> class but knows many
 * more aliases and is compatible with Java 1.3. It will use a simple detection
 * mechanism to detect what character sets the current VM supports. This will
 * be a sub-set of the character sets listed in the
 * <a href="http://java.sun.com/j2se/1.5.0/docs/guide/intl/encoding.doc.html">
 * Java 1.5 (J2SE5.0) Supported Encodings</a> document.
 * <p>
 * The <a href="http://www.iana.org/assignments/character-sets">
 * IANA Character Sets</a> document has been used to determine the preferred
 * MIME character set names and to get a list of known aliases.
 * <p>