[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.
Code has been updated as your suggestions, and right now we don't need to take care about cookie/session, as long as we pass cookieString into instance via ```acsApp.setSessionByCookieString(cookieString)``` after user's login.
Also, we can use ```var acsApp = require('acs-node')(ACS_APPKEY, {cookieString: req.session.cookieString})``` to instantiate an ACS Node object.
Please refer to updated readme from here: https://github.com/realpaul/acs-node-sdk/tree/8fe63bf24693936a200ea61c38ea0e73abbed1e0#use-acs-node-sdk-directly