added StandardErrorCallback class plus errorCallback field for config

[stokpop]

The StandardErrorCallback class can be registered via the errorCallback field in log4j/logback/java-logging config.
It logs to std err.

This is for issue 214.

Or custom errorCallbacks can be registered.

When using the StandardErrorCallback the following property can be used to also log stack traces:

-Dcom.splunk.logging.util.StandardErrorCallback.enablePrintStackTrace=true

Note this PR also includes a little change working with com.splunk.logging.HttpEventCollectorErrorHandler.ServerErrorException: it seemed not to make sense to wrap regular exceptions (e.g. non-Splunk errors that have json message with error code) inside a ServerErrorException.

[vmalaviya-splunk]

@stokpop PR looks good.
One minor change, could you please remove the version change info from the README and CHANGELOG?

Could you also elaborate on why the custom-exception (HttpEventCollectorErrorHandler.ServerErrorException) has been removed and instead general-exception (Exception) is used?

[stokpop]

@vmalaviya-splunk thanks for reviewing the PR, I have reverted the changes in README and CHANGELOG.

The (HttpEventCollectorErrorHandler.ServerErrorException) is still there and used where appropriate: for Splunk server errors, where a json error message is returned and used as input to the ServerErrorException. When errors occur like TimeoutOutExceptions, that are not errors returned from the Splunk server, it does not make sense to use the ServerErrorException, imho. See for instance the code in: com.splunk.logging.HttpEventCollectorSender#postEventsAsync, the completed() method deals with server errors with json message, but the failed() method has only Exception and no json to use.