[NODEJS-1598] PoC of acs-node-sdk Rework

## Ticket Information
***JIRA Ticket***
https://jira.appcelerator.org/browse/NODEJS-1598

***Description***
This PR is about to raise a PoC of ACS Node SDK rework. It re-designs framework of acs-node-sdk, and supports multiple instances. It also provides a way to list all available ACS objects and methods including doc url for a good reference. Test framework with travis-ci is added too for a good and automatic test.

Since this is only a PoC, I only implemented ACS object "Users". If this way is towards to a good direction and passes code review, we will add all other ACS objects later.

## Get started
Please refer to readme file https://github.com/realpaul/acs-node-sdk/blob/NODEJS-1598_ReworkPoC/README.md for a general usage.
Thanks Jeff and Neeraj, and I have added custom error handling part with the latest commits. Please refer to [lib/acs_error.js](https://github.com/realpaul/acs-node-sdk/blob/4b2ce65701a5bd0486be8b8540492b2eab1b15e5/lib/acs_error.js) and its caller [lib/acs.js#L39-L42](https://github.com/realpaul/acs-node-sdk/blob/4b2ce65701a5bd0486be8b8540492b2eab1b15e5/lib/acs.js#L39-L42) to get it.
All error messages with error code are stored in [lib/messages.js](https://github.com/realpaul/acs-node-sdk/blob/4b2ce65701a5bd0486be8b8540492b2eab1b15e5/lib/messages.js).