Add service script for CentOS
Add install script for CentOS
Add droonga-engine.yaml for CentOS
Merge pull request #24 from long-long-float/install-script-for-centos

Install script for CentOS
Specify log_path as a relative path from the base directory
Don't specify "trace" level for non-developer environments!
Specify log_path as a relative path from the base directory
Don't specify "trace" level for non-developer environments!
Specify default host, port and tag
Specify default host, port and tag
More descriptive error message for not supported platforms
Make Session loggable
Make WorkerProcessAgent loggable
Log start/done of on_finish callback for Engine#stop_gracefully
Log session id
Log start/done for WorkerProcessAgent#start/stop
Output more logs for debugging of #11 in the trace level
Add license header
Extract common part to setup configuration directory
Don't install needless package.

"npm" is required by droonga-http-server, not droonga-engine.