[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 Simon, and I have separated functions for test purpose only in util.js, and moved them into test/testUtil.js. Please refer to [util.js](https://github.com/realpaul/acs-node-sdk/blob/8fe63bf24693936a200ea61c38ea0e73abbed1e0/lib/util.js) and [testUtil.js](https://github.com/realpaul/acs-node-sdk/blob/8fe63bf24693936a200ea61c38ea0e73abbed1e0/test/testUtil.js).