Implement unregisterDevice method

[baldercm]

Sending a PR with new unregisterDevice capabilities. Usage would look like this:

var Pushwoosh = require('pushwoosh-client');
var client= new Pushwoosh('AppCode', 'AuthToken');

var unregisterDeviceOptions = {
    hwid: 'hwid'
};

// this will unregister the device from the client's 'AppCode' application
client.unregisterDevice(unregisterDeviceOptions, function(error, response) {
     ...
});