supporting synchronized call and fix running in child thread

[z-hao-wang]

see readme examples

[z-hao-wang]

this also fixes context issue when running in child processes.
nodejs/node#21481

[danielsokolowski]

This looks good to me, can it be merged in?

[z-hao-wang]

@danielsokolowski the example are in the readme, you can avoid using a callback.
The original purpose is that I have to work with a different lib that doesn't support callback type calls.
Also this enables to run in forked child thread.

[oransel]

May be the sync functions need to be separate than the async functions so we don't loose the async functionality

[z-hao-wang]

@oransel I believe this change is fully backward compatible, it will use async method if you pass in a callback, otherwise it will do sync call. how else could we separate the sync methods?

[oransel]

@z-hao-wang Sounds good. If you can resolve the merge conflicts, i can pull in and test.

[z-hao-wang]

@oransel Just merged conflicts and test on my side worked.