Adding support for promises API --- chmod

[Peng-Qu]

add a test for function: chmod if the path does not exist, it should return err if the mode dose not pass the modecheck function, it should return err if all go though, the access permission of the file will be changed.

[humphd]

OK, go for it @Peng-Qu.

[danielbogomazov]

Dupe of issue 405

[danielbogomazov]

I've done everything except for the case where the path does not exist. If you'd like, I can push my changes as I have them right now and you can contribute the path does not exist check to it? Let me know whenever you can.

[Peng-Qu]

sorry for late. it's Ok. I will do the path does not exist on 'chmod'. Thank you.

[danielbogomazov]

Sounds good. Here's the pull request