[NGRINDER-483] Modify the message key
- Simplfy the healthcheck method
[NGRINDER-494] Increase default vuser size to 300
[NGRINDER-495] Modify process and thread calc to support vuser more than
600.
[NGRINDER-479] 
- add json lib on agent
- not distibute logaback-worker when ngrinder.dist.logback is set
- make heartbeat faster.
[NGRINDER-486] Make agent is updatable by updata message.
- Not finished.
[NGRINDER-496] 
- locate our security manager in the head of classpath
- Make script not to link the native lib
[NGRINDER-489] Make the timeout logger to 6000
[NGRINDER-497] Make it stable..
[NGRINDER-486] Make agent status to update when updating.
[NGRINDER-496] Make user not link native lib as well.
[NGRINDER-497] Make user can configure the local ip
[NGRINDER-487] Fix typo errors
Prepare 3.1.1 official release.