[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.
Yes and code has been updated. Will follow this style in the future.
But please note const is not recognized by JSHint unless we enable [ECMAScript 6 specific syntax](http://www.jshint.com/docs/options/#esnext) in .jshintrc. Before that we avoid using this because it is not an official supported keyword by some IDEs like Eclipse.