Support for WRAP tokens

This PR aims to **support to use WRAP tokens, which will allow to consume VSO REST API using service account identities**.

Summarizing, this PR adds:
  - **a new method getWrapToken** to get a new WRAP token from ACS for the specified account URL, username, and password
  - **a new method  createWrapClient** to create a VSO client instance based on a WRAP token

Here is a code snippet illustrating the usage of these new methods 

```javascript
var vsoClient  = require('./public/vso-client.js');
var util = require('util');

var accountUrl = 'https://your_account_name.visualstudio.com';
var username = 'your_account_service_username;
var password = 'your_account_service_password;

vsoClient.getWrapToken(accountUrl, username, password, function (err, data, resp) {
    if (err) {
        console.log('Error getting a WRAP token:' + util.inspect(err));
        return;
    }

    var accessToken = data.wrap_access_token;
    var client = vsoClient.createWrapClient(accountUrl, 'DefaultCollection', accessToken);
    //Now you can use the client as usually
    client.getProjects(function (err, data) {
        if (err) {
            console.log('Error getting a projects:' + util.inspect(err));
            return;
        }
        console.log('Here are your projects:\n' + util.inspect(data));
    });
});
```