Updated the AbstractLogglyAppender to have another method that
implementing classes must provide: getEndpointPath() that returns the
URL path for the Loggly service (inputs/ for the standard HTTP/S service
and bulk/ for the bulk HTTP/S service).

This prevents the user from needing to manually override the endpoint
URL for the LogglyBatchAppender in order to take advantage of the bulk
logging API (where a single HTTP request can contain multiple log
events, which will be parsed out accordingly).