Fix some prblems:
    1.Handle the EsRejectedExecutionException[rejected execution (queue capacity 50)
    2.Chinese string may be truncated.
    3.Decrease the http.globalAgent.maxSockets to avoid too many
    EsRejectedExecutionException
Merge branch 'master' into pr/56

Conflicts:
	drivers/es.js
- Improved doc. Fixes #57