Add error callback via logger configs and add default error callback implementation

[stokpop]

As a user of the splunk-library-javalogging jar we would like to see and register errors when calls to the Splunk HEC appender fail and for what reason. For instance, when Splunk is not available or cannot parse or accept log events or when timeouts occur.

Create a default error callback that logs to standard error to be used out of the box.

Add the possibility to register an error callback implementation via log4j/logback/java-logger config.
You can do so, for instance via logback.xml, by adding this to the appender xml:

<errorCallback>com.splunk.logging.util.StandardErrorCallback</errorCallback>

Optionally generate stack traces when errors occur.

(Submitted a working PR: #215)

[bparmar-splunk]

HI @stokpop,
I am closing this issue as its related PR is merged.