README update

The readme refers to the wrong GA variable, you want to be calling login on the instance, not the GA "class/factory"

```
[orta] node
> var GA = require('googleanalytics'),
...     util = require('util'),
...     config = {
...         "user": "myusername",
...         "password": "mypassword"
...     },
...     ga = new GA.GA(config);
undefined
> ga
{ user: 'myusername',
  tokenType: 'GoogleLogin',
  password: 'mypassword',
  token: null }
> ga.get
[Function]
> GA.get
undefined
> GA.login
undefined
```