Added support for Loggly batch uploads

Loggly supports batch processing of log messages via the bulk API (https://www.loggly.com/docs/api-sending-data/#bulk).

This patch for `LogglyHandler` - when used in conjunction with `FingersCrossedHandler`, `BufferHandler`, etc. - adds support for sending multiple messages to Loggly in a single request.

The generic `JsonFormatter` cannot be reused as Loggly expect batch logs to be uploaded as JSON-encoded strings with each event separated by new lines.