AMQP-282 Log4j Appender - Configurable Charset

A charset is used when converting the message body
String to a byte[] to send to AMQP.

For backwards compatibility, the appender uses the
system charset by default, but now allows the charset
name to be specified in the log4j configuration...

log4j.appender.amqp.charset=UTF-8

__cherry-pick to 1.1.x__