[cyb-59] JS fetch auth library

JS fetch auth library implementation.

According to @anwar6953 comments next changes are implemented:
1. Browser and NodeJS logic separated into different files;
2. Documentation improved;
3. Minor code corrections.

Files not required to be reviewed( dependecies for tests that could not be installed by 'npm' or 'bower' ):
1. cdap-authentication-clients/javascript/src/base64.js;
2. cdap-authentication-clients/javascript/test/formatio.js;
3. cdap-authentication-clients/javascript/test/samsam.js;
4. cdap-authentication-clients/javascript/test/sinon-server-1.10.3.js.
In order to randomly select a url from the array, simply do: 
```
Math.floor(Math.random() * (authUrls.length))
```
The current implementation in this PR is biased. For instance, when authUrls.length is 2, only the authURL at index 1 will ever be returned.