Cadence java client release v2.6.0 

Prepare for release 2.5.2 (cadence-workflow#343) 

Prepare for 2.5.1 release (cadence-workflow#339) 

Prepare for 2.5.0 release (cadence-workflow#327) 

Prepare for release 2.4.2 (cadence-workflow#314) 

Prepare 2.4.1 release (cadence-workflow#276) * Prepare 2.4.1 release * Update readme

Cron child workflow (cadence-workflow#264) * Add cron schedule to child workflow option * Fix unit test * Review comments

Expose methods to allow user to add headers in Thrift request (cadenc… …e-workflow#255) * Expose methods in TChannel service to allow user to add headers in Thrift request * Update changelog

Fix retryer in activity and workflow worker (cadence-workflow#250) * Do not retry certain errors on respond activity/workflow completion and failure * Rebase, bump up version and update changelog

Fixed JsonDataConverter to correctly report non serializable exceptio… …ns (cadence-workflow#238)