[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 the case that authentication isn't enabled, this doesn't cache the value (and so multiple http calls are made to in fetchAuthUrl()). Please make it cache the value.

Here's an example of it caching this value, so it only calls fetchAuthUrl once:
https://github.com/caskdata/cdap-clients/blob/develop/cdap-authentication-clients/java/src/main/java/co/cask/cdap/security/authentication/client/AbstractAuthenticationClient.java#L76